Basin analysis study of block 10 in the Say'un-Masilah Basin,.pdf

ORIGINAL PAPER

Basin analysis study of block 10 in the Say'un-Masilah Basin,Yemen, using a 1D backstripping method

Adel Mohammad Al-Matary & Hilal M. Ahmed

Received: 2 November 2010 /Accepted: 3 January 2011# Saudi Society for Geosciences 2011

Abstract Geohistory analysis was carried out on block 10in the Say’un-Masilah Basin. The present study is based onthe analysis of the well logs of six exploration wells. In theMiddle Jurassic to the Middle Eocene, Say’un-MasilahBasin exhibited a complex subsidence history over a periodof about 155 Ma. Backstripped subsidence curves areconstructed by removing the effects of decompaction to thewater column and sediment loads. In the Middle Jurassic,slow subsidence was driven under the effect of sedimentload as Kuhlan and Shuqra Formations were deposited aspre-rift deposits. The average total subsidence value of thebasement during the deposition of Shuqra Formation was276 m. The highest subsidence rates during this time areobserved northeast of the study area. Rapid subsidenceinitiated in the Upper Jurassic driven by mechanicalextension of the rift, resulting in the deposition of Madbiand Safer Formations, or the so-called Syn-rift deposits.The average tectonic subsidence value of the basementsduring the deposition of Madbi Formation was 368 m. Thehighest subsidence rates during this time are observedsouthwest of the study area. Mechanical extension ceased atabout 145 Ma, being replaced by a phase of post-riftsubsidence, resulting in more widespread uniform sedimen-tation, with reduced tectonic subsidence rates forming post-riftdeposits represented by Nayfa Formation, Sa’ar Formation,Biyad Formation, and Mahra Group. The total subsidencevalue of the basement during the deposition of the post-riftsediments increases gradually due to the increase of thesediment load as the rate of deposition was high especiallynortheast of the area.

Keywords Yemen .Masilah basin . Backstripping .

Geohistory . Basin analysis

Introduction

The Republic of Yemen is geographically located at thesouthern tip of the Arabian Peninsula, situated betweenlatitudes 12°40′–18°50′ N and longitudes 42°50′–53°00′ E.Geologically, Yemen is located on the southwestern cornerof the Arabian plate opposite the Afar depression.

Redfern and Jones (1995) subdivided the interior Yemeninto the following major tectonic units: basement highs,Jurassic rift basins, Cretaceous rift basins, and intergrabenhighs and marginal terraces. The Late Jurassic riftingbetween East Africa and West India also caused extensionaltectonics in Yemen, and sediments beginning to fillMesozoic grabens thus developed, including the Sabatayn,Say’un-Masilah, Balhaf, Siham-Ad-Dali’, and Jiza’-Qamarbasins (Fig. 1).

The two Mesozoic basins of the Sabatayn and theSay’un-Masilah are currently the only hydrocarbon-producing basins in Yemen. Both of them are the twomajor Jurassic rift basins within interior Yemen, which areflanked and separated by major basement highs.

The studied block 10 is located in the northern centralpart of Say’un-Masilah Basin (Figs. 1 and 3) that has longbeen the site of hydrocarbon exploration and production.However, few studies have investigated their thermal andburial histories, and the timing of hydrocarbon generation istherefore poorly constrained.

Backstripping is a quantitative method of estimatingtectonic subsidence, which is defined as the vertical move-ment of basement in the absence of both sediment loading andsea level change (Watts and Ryan 1976). It is routinely

A. M. Al-Matary (*) :H. M. AhmedEarth & Environmental Sciences Department, Sana’a University,Sana’a, Yemene-mail: [email protected]

Arab J GeosciDOI 10.1007/s12517-011-0280-0

performed on 1D borehole or cross-section stratigraphy (e.g.,Scheck and Bayer 1999; Van Sickel et al. 2004; Carminati etal. 2007) and on 2D cross-sections drawn on seismicreflection data (e.g., Steckler et al. 1999; Roberts, et. al1998 and references therein; Ceramicola et al. 2005).

The goal of this study was to produce a representation ofthe vertical movement of a stratigraphic horizon in Say’un-Masilah Basin as an indicator of subsidence and uplifthistory of the basement in the basin since sedimentarycover is being deposited.

Geology and stratigraphic frameworkof the Say’un-Masilah Basin

The Late Jurassic rifting between East Africa and WestIndia caused extensional tectonics in Yemen (but notablywithout rift-related volcanism), and sediments beginning to

fill Mesozoic grabens thus developed, including theSabatayn and Say’un-Masilah basins. Both have a predomi-nant NW–SE orientation and are delimited bymajor boundingfaults, which probably reflect reactivation, or controlled bypreexisting Najd trend faults. These well-explored basins arefilled with syn-rift and post-rift sediments that share manysimilarities in source and reservoir rocks, but the Tithonianevaporate is absent in Say’un-Masilah Basin.

The thickness of the sedimentary succession in theMesozoic basins is between 1.8 and 3 km in the western partsand 4–6 km in the east parts of Yemen (As-Saruri et al. 2010).

The Say’un-Masilah Basin, as its name implies, isdivisible into two sectors. The western part of Say’unsector is more rift-like in its configuration and subsidencemanner than the eastern or Masilah sector, where depositshave intertongued more with open marine conditions.

Lithostratigraphic units in the basin have been studied bymany authors, including Haitham and Nani (1990),

Fig. 1 Map of the outcrop geology, major structural highs, and sedimentary basins of Yemen (after As-Saruri et al. 2010 with modifications)

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Bosence et al. (1996), Bosence (1997), Putnam et al.(1997), Beydoun et al. (1996, 1998), Beydoun andAs-Saruri (1998), Watchorn et al. (1998), and Cheng et al.(1999). The most current detailed stratigraphic and basinnomenclature of Yemen is provided by (Beydoun et al.1998). As backstripping study may be the first in this basin,another one is carried out in the Sabatayn Basin (Al-Matary2007). Recent active oil exploration works in Say’un-Masilah Basin revealed the presence of a subsurfaceJurassic to Recent stratigraphic column (Fig. 2). It can besummarized below.

The pre-rift sedimentary sequences in these basins arerepresented by the Kuhlan Formation (Bathonian/Callovian)and the Shuqra Formation (Oxfordian). Upper Jurassic–EarlyCretaceous syn-rift sediments in the Mesozoic basins consti-tute the predominant petroleum plays and production inYemen. Early syn-rift sediments of the Mesozoic basins arerepresented by the Madbi Formation which consists in thesurface section of organic-rich marine shale (Lower andUpper Madbi Shales Members), debris flow and breccias(Rafad Member), and well-bedded limestone (Ma’abirMember). In the subsurface section, the Madbi Formation

is divided into the rich bituminous shale of the MeemMember in the lower part and the shale and turbidite ofthe Lam Member in the upper part .The turbidities of theMadbi Formation are overlain by the thick evaporites ofthe Sabatayn Formation. The Barremian-aged QishnClastics offer another noteworthy reservoir rock, espe-cially in the Say’un-Masilah Basin. These sandstoneshave high porosity and permeability and are free frominhibiting authigenic clay or other diagnetic problemsbecause of their quartz-rich composition and relativelyshallow burial depth (Putnam et al. 1997; Leckie andRumpel 2003; King et al. 2003).

The Madbi Formation is overlain by platform carbonatesof the Nayfa Formation (Upper Tithonian) and deepwaterlimestone and shale sediments of the Sa’ar Formation(Berriasian–Valanginian). The remaining Cretaceous sedi-ments (mainly shallow marine limestones, marls, andsandstone) in the Mesozoic basins are sandwiched betweenmajor unconformities at the Hauterivian, Aptian, Cenoma-nian, and Danian stages, reflecting transgressions due toeustatic changes or non-deposition/erosion by tectonicuplifts.

Fig. 2 General columnar sec-tion for the stratigraphic unitsin the Say’un-Masilah Basin

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The Say’un-Masilah Basin is characterized by numerousunconformities which surfaced due to volcanic activationand deposition of the Jurassic sediments directly on thebasement rocks.

At bock (10), the sedimentary basin fills ranges inthickness from 4,000 m in the deep centers to 1,800 m onthe structural highs.

The Kuhlan Formation, Amran Group, Sa’ar Formation,Mahra Group, Hadramawt Group, and Shihr Group are themain stratigraphic units in the Say’un-Masilah Basin (Fig. 2).

Materials and method

This paper study is based on the data of six boreholesdrilled in block 10 in the Say’un-Masilah Basin. These areKharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells (Fig. 3and Tables 1 and 2). These data are used for theachievement of the stratigraphic setting in the study areaand to determine the porosity of each rock unit.

The backstripping method is used for studying thegeohistory of the Say’un-Masilah Basin. It is a techniqueused on basin studies such as geohistory analysis thatseparates isostatic effects of sediment and water loadingfrom those of tectonic subsidence (Steckler and Watts 1978;Van Hinte 1978). It is a method by which successive layersof basin fill sediment are “stripped off” the total stratigraphyduring analysis of that basin’s history.

Mathematical procedures to calculate tectonic subsi-dence through time from stratigraphic columns have beendiscussed, for instance, by Steckler and Watts (1978) Allenand Allen (2005). This analysis yields a series of curvesthat explain the history of sediment accumulation andsubsidence through time as indicated by a single verticalstratigraphic column via well log or a measured section; thecurves can be compared from place to place within a basin,or from a basin to another basin. This analysis requiresknowledge of the stratigraphic column showing the present-day thickness of the stratigraphic units, types of lithologies,ages of horizons, and estimated paleowater depths. This

Fig. 3 Location of study area and the selected wells

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analysis also requires information about the vertical positionof the depositional systems relative to some data, bathym-etry or topography and absolute sea level.

Geohistory diagrams show the interaction betweensediment supply, eustatic sea level changes, and basementsubsidence through time. Subsidence can be driven bytectonic movement, compaction of sediment under its ownweight, or some combination of both.

Geohistory analysis of block 10 in the Say’un-MasilahBasin

The aim of backstripping is to analyze the subsidence history ofa basin by modeling a progressive reversal of the depositionalprocess. While backstripping may be applied to any sedimen-tary basin (including platforms and foreland basins), it is mostcommonly applied to extensional basins where it is used todetermine the magnitude of lithosphere stretching from post-rift subsidence rate (Sclater and Christie 1980).

In this paper, the history of vertical tectonic motionsof the basement rocks in block 10 is quantified usingairy (1D) isostasy backstripping and tectonic subsidencemodeling analysis. There are basically three steps inconstructing a subsidence or geohistory curves (Athy1930; Van Hinte 1978). The first is determining thesediment thickness and porosity for each unit, thenmaking the correction of the compaction effect toreconstruct the original thickness of the unit, and finallycalculating the amount of subsidence and sedimentationrates.

1. Determine sediment porositiesThere are a lot of different ways to calculate

sediment porosity, but most of them rely on theassumption that porosity decreases with burial depth.In this study, the porosity of the studied formations isderived from well logs and Eq. 1.

fN ¼ foe�cz ð1Þ

2. Sediment decompactionThe first step in backstripping is to reconstruct the

original sediment thicknesses (TO) of the growingsedimentary fill from the basin floor up-to-date strati-graphic boundaries in particular exposures or well logs.Provided a sedimentary column with the presentthickness (TN), the present-day porosity (∅N), and theoriginal porosity (∅O), thus the original thickness forany unit (TO) is given by:

TO ¼ 1� fNð ÞTN1� fOð Þ : ð2Þ

Table 1 shows the names, total depth, and elevations of the selectedwells

Well name Well symbols Total depth (m) Elevation abovesea level (m)

Kharir 1-37 KHA 1-37 2,953 956.88

Kharir 1-25 KHA 1-25 2,882.5 1000.9

Kharir 1-11 KHA 1-11 2,350 989.5

Kharir 1-15 KHA 1-15 2,580 942.5

Kharir 2-20 KHA 2-20 3,069 951.5

Kharir 1-16 KHA 1-16 2,550 910.5

Table 2 Names of stratigraphic units and the present-day depth to the top of these Formations (in km) at each well

Unit ID Formations KHA 1-37 (km) KHA 1-25 (km) KHA 1-11 (km) KHA 1-15 (km) KHA 2-20 (km) KHA 1-16 (km)

1 UMM ER RASHUMAH 0 0.015 0.015 0.015 0 0.014

2 SHARWAYN 0.26 0.307 0.308 0.26 0.269 0.27

3 MUKALLAH 0.293 0.326 0.338 0.295 0.287 0.29

4 FARTAQ 0.819 0.8595 0.858 0.827 0.78998 0.875

5 HARSHIYAT 0.853 0.894 0.883 0.862 0.82298 0.913

6 UNCONFORMATY 1.551 1.667 1.63 1.593 1.62184 1.575

7 QISHN 1.551 1.667 1.63 1.593 1.62184 1.575

8 BIYAD 1.666 1.774 1.73 1.69 1.73034 1.687

9 UNCONFORMATY 1.964 2.093 1.978 1.965 2.04882 1.983

10 SA’AR 1.964 2.093 1.978 1.965 2.04882 1.983

11 NAYFA 2.429 2.529 2.251 2.345 2.49925 2.4

12 MADBI 2.523 2.604 2.27 2.37 2.64659 2.432

13 SHUQRA 2.778 2.8175 2.322 2.562 2.78927 2.48

14 KUHLAN 2.948 2.8782 2.35 2.573 3.06982 2.55

15 BASEMENT 2.953 2.8825 2.35 2.58 3.06982 2.55

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3. Sediment accumulation ratesThe rate of fill (Rf) at which the sedimentary rock

accumulated during any interval of time can be calculated.If the time span (A) of the interval is expressed inmegayears and the original thickness (TO) in meters, then

Rf ¼ TO10A

cm=1; 000 years: ð3Þ

4. Total subsidenceThe total depth of a basin is the sum of the thickness

of the sediments and the water depth above that basinfill. So we can calculate total subsidence as follows:

S ¼Xi

1T

» þWDi ð4Þwhere S is the total subsidence, WD is the water depth,and T* is the decompacted thickness.

5. Tectonic subsidenceThe tectonic subsidence of the basement rocks can

be calculated from the general backstripping equationof Steckler and Watts (1978):

Zi ¼ T» þ ra � rs

ra � rwþWDi ð5Þ

where Zi is the depth to the tracked horizon relative to sealevel (i.e., the amount of tectonic subsidence); ρa is thedensity of the asthenosphere (3,330 kg/m3); ρs is thedensity of the sediment column; ρw is the density of water(1,030 kg/m3); and WDi is the water depth for unit i. T*is the decompacted thickness. The density of thesedimentary column (ρs) changes as the thickness of thestratigraphic section changes due to compaction; it mustbe calculated after each new unit is deposited. FollowingSteckler and Watts (1978), ρs can be calculated for thecolumn after the deposition of unit i by this equation:

r si¼Pi

i¼1 firw þ 1� fið Þrgh i

T»i

S» ð6Þ

where S» ¼ Pi

1 T»i where ρg is the grain density of unit

and ∅i is the porosity of unit i. values for grain densityand the maximum surface porosity ∅O are shown inTable 1.

6. Subsidence rateIt can be calculated by dividing the total subsidence

(S) in meters by age (Ma, Ka, or year) of any unit.

Rs ¼ S

10Acm=1; 000 years: ð7Þ

Input parameters for airy (1D) backstripping

The ages of the chosen stratigraphic horizons (Table 2) inthe studied wells were derived from the “International

Lexicon of Stratigraphy” (Beydoun et al. 1998). No specificpalebathymetric data were available; instead, the standardpaleowater depth of common lithology is used (Fluegel2004).

FLEX DECOMP (Badley’s Company 2000) is a soft-ware that deals with the problem of removing a laterallyvarying load from a lithosphere exhibiting finite strength bypoint sampling the cross-section at a regular interval. Thisprogram is used here to calculate some parameters likeconstant, C), initial porosity ∅O, and density of units, ρs(Table 3).

Six deep exploration wells (Tables 1 and 2) wereselected to calculate the total subsidence of the basementrocks in the (block 10) in Say’un-Masilah Basin and plot itagainst the age to the base of the studied formation in eachwell (Figs. 4, 5, 6, 7, 8, and 9).

Results and discussion

The resulted subsidence and sedimentation rates for allunits are shown in Tables 4, 5, and 6 and will be discussedfor each formation separately as follows.

Kuhlan Formation

It is the lowest formation of the stratigraphic section in thestudied block. The Kuhlan Formation consists of coarse- tofine-grained sandstones locally interbedded with non-calcareous silts and shales. It rests unconformably on thebasement rocks in most of the studied well. The age to thebase of this formation is 165 Ma. Its initial porosity is 49%calculated using Flex-Decomp software, and the averagepresent-day porosity is 23% obtained from the well logs.

Kuhlan Formation has a thickness of about 5, 4.3, and7 m, in Kharir 1-37, 1-25, and 1-15 wells, respectively. Thehighest rate of deposition for this formation is 1.4 m/Ma,which was obtained from Kha 1-25, and the lowest one is0.86 m/Ma, obtained from Kha 1-15.

The decompacted thickness due to the sequentialremoval of compaction effects of this formation in thestudied wells are 8, 7, and 10 m in Kharir 1-37, 1-25, and 1-15 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest effect of compaction on this formation is obtainedfrom Kha 1-15 well.

The tectonic subsidence values of the basements duringthe deposition of Kuhlan Formation, calculated using the1D method, are 7, 4, and 7 m in Kharir 1-37, 1-25, and 1-15wells, respectively (Table 3), the highest tectonic subsidencerates being observed in Kharir 1-15.

The total subsidence values of the basement during thedeposition of Kuhlan Formation, calculated using the 1Dbackstripping method are 8, 7, and 10 m in Kharir 1-37, 1-25,

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and 1-15 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thesedimentation rate of this formation in this region was low.

Shuqra Formation

The Shuqra Formation is the basal formation of the AmranGroup, and it predominantly consists of carbonates deposited

in a shallow marine neritic environment of a wide inner shelf.The carbonate succession of the Shuqra Formation is highlyfossiliferous, well-bedded, and commonly oolitic limestones(sandy at base), with interbedded marls, dolomites, shales, andoccasional sandstones (Redfern and Jones 1995; Beydoun etal. 1998). The age to the base of this formation is 154 Ma. Itsinitial porosity is 70% calculated using Flex-Decomp

Fig. 4 Matrix calculation of the decompacted thickness and initial porosities of the studied formation and the subsidence curves obtained fromKharir 1-37 well in block 10

Unit name Age to the base (Ma) ∅O ∅N C ρ (km)

UM ER RADHUMAH 48 0.7 0.63 0.71 2,710

SHARWAYN 65 0.6447 0.55 0.55 2,720

MUKALLAH 70 0.5181 0.42 0.35 2,660

FARTAQ 93 0.6615 0.39 0.6 2,720

HARSHIYAT 96 0.5345 0.35 0.35 2,670

UNCONFORMATY 108 0 0 0 0

QSHIN 112 0.6545 0.25 0.58 2,720

BIYAD 130 0.5338 0.28 0.35 2,670

UNCONFORMATY 135 0 0 0 0

SA’AR 137 0.6755 0.15 0.64 2,710

NAYFA 142 0.6772 0.14 o.64 2,710

MADBI 147 0.6825 0.13 0.66 2,710

SHUQRA 155 0.7 0.10 0.71 2,710

KUHLAN 167 0.49 0.23 0.27 2,650

Table 3 Required geohistoryparameters used to calculatethe subsidence rate of thebasement at each depositionstage of new unit

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software. The average present porosity of Shuqra Formationis 10% obtained from the well logs. The present thickness ofShuqra Formation in the selected wells are a 170, 61, 28, 11,281, and 48 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively. The highest average rate of depositionfor this formation is 2.43 cm/1,000 years, observed in Kharir1-37 south of the study area, and the lowest one is0.16 cm/1,000 years, obtained from Kharir 1-11 in the north.The decompacted thickness due to the sequential removal ofcompaction effects of this formation in the studied wells are453, 175, 134, 32, 699, and 195 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and1-16 wells, respectively (Figs. 4, 5, 6, 7,8, and 9). The highest effect of compaction on this formationis obtained from Kha 2-20 well where about 415 m ofsediment is compacted under effect of sedimentary loadingabove this formation and tectonic subsidence. The tectonicsubsidence values of the basements during the deposition ofShuqra Formation, calculated using the 1D method, are348.2, 139.8, 61.87, 31.85, 518.5, and 151 m in Kharir 1-37,1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively (Table 5);the highest tectonic subsidence rates are observed in Kharir2-20 in the northeast of the study area.

The total subsidence values of the basement during thedeposition of Shuqra Formation, calculated using the 1Dbackstripping method are 460, 181, 79, 42, 699, and 195 m

in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells,respectively (Figs. 4, 5, 6, 7, 8, and 9). The highestsubsidence rates are observed in Kharir 2-20 northeast ofthe study area.

Madbi Formation

It deposited conformably overlying the Shuqra Formation.The formation is generally rich in organic content and isconsidered as the principal source rock for all the hydro-carbons so far discovered in the Sabatayn and Say’un-Masilah basins (Beydoun et al. 1998). It consists of about76% limestone and 24% shale. The age to the base of thisformation is 150 Ma. The initial porosity of the formation is68% calculated using Flex-Decomp software, and theaverage present porosity is 13% obtained from the welllogs. Its present thickness in the selected wells are 255, 214,52, 192, 143, and 48 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively. The highest average rate ofdeposition for this formation is 6.4 cm/1,000 years, observedin Kharir 1-37 south of the block, and the lowest one is 1.2 cm/1,000 years, obtained from Kharir 1-16 in the east.

The decompacted thickness are 603, 519, 134, 462, 363,and 126 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). The highest


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effect of compaction on this formation is observed in Kha1-37 well where about 348 m of sediment is compactedunder effect of sedimentary loading above this formationand tectonic subsidence.

The tectonic subsidence values of the basements during thedeposition of Madbi Formation are 592.2, 363, 149.5, 363,536.2, and 206.9 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest tectonic subsidence rates are 592.2 m in Kharir 1-37southwest of the study area.

The total subsidence values of the basement during thedeposition of Madbi Formation, calculated using the 1Dbackstripping method are 862, 626, 198, 491, 806, and280 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16wells, respectively (Table 6). The highest subsidence ratesare observed in Kharir 1-37 southwest of the study area.

The Madbi Formation is deposited during the initial andmain periods of Jurassic rifting. The relatively confined natureof the Jurassic basins that developed in Yemen is reflected in adistinctive restricted facies of the Madbi within these basins.

Nayfa Formation

It is overlying Madbi Formation with conformable contact.The Nayfa Formation exhibits two different facies withinSay’un-Masilah Basin. One of these facies was deposited in

a more restricted but pelagic rift setting and characterizedby porcellanites, condensed sequences, and calci-turbidites,whereas the other is a pelagic but more open-marine facies.The age to the base of this formation is 144 Ma. Its initialporosity is 67% and the average present porosity is 14.3%.The present thickness of Nayfa Formation in the selectedwells are 94, 75, 19, 25, 147, and 32 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively. The highestaverage rate of deposition for this formation is2.45 cm/1,000 years. It is observed from Kharir 2.20 north ofthe study area. The lowest one is 0.31 cm/1,000 years, which isobtained from Kharir 1-11 northwest of the study area. Thedeposition of the Nayfa Formation took place during a newphase of transgression probably associated with the erosion ofthe marginal barriers that were present duringMadbi times thatresulted in a partial reintroduction of open-marine sedimenta-tion into Say’un-Masilah Basin (Redfern and Jones 1995).

The decompacted thickness in the studied wells are 236,162, 49, 65, 362, and 83 m in Kharir 1-37, 1-25, 1-11, 1-15,2-20, and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9).The highest effect of compaction on this formation isobserved in Kha 2-20 well where about 215 m of sedimentis compacted under the effect of sedimentary loading abovethis formation and the tectonic subsidence. The tectonicsubsidence values of the basements during the deposition ofNayfa Formation are 592.2, 458.3, 170.5, 329.8, 641.5, and


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243.6 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16wells, respectively (Table 5). The highest tectonic subsidencerate is 641.5 m in Kharir 2-20 northeast of the study area.

The total subsidence values of the basement during thedeposition of Nayfa Formation are 919, 678, 230, 472,1,001, and 335 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). The

highest subsidence rate is observed in Kharir 2-20 northeastof the study area.

Sa’ar Formation

This formation represents the basal sequence of theCretaceous successions. It is unconformably overlain by


Table 4 Rate of fill (Rf) or the rate of sedimentation for each formation in the studied wells

Formations Kha 1-37(cm/1,000 years)

Kha 1-25(cm/1,000 years)





UMM ER RAdHUMAH 1.53 1.72 1.72 1.44 1.58 1.51

SHARWAYN 0.66 0.38 0.60 0.70 0.36 0.40

MUKALLAH 2.29 2.32 2.26 2.31 2.19 2.54

FARTAQ 1.13 1.15 0.83 1.17 1.10 1.27

HARSHIYAT 5.82 6.44 6.23 6.09 6.66 5.52

UNCONFORMATY 0.00 0.00 0.00 0.00 0.00 0.00

QISHN 0.64 0.59 0.56 0.54 0.60 0.62

BIYAD 5.96 6.38 4.96 5.50 6.37 5.92

UNCONFORMATY 0.00 0.00 0.00 0.00 0.00 0.00

SA’AR 9.30 8.72 5.46 7.60 9.01 8.34

NAIFA 1.88 1.50 0.38 0.50 2.95 0.64

MADBI 3.19 2.67 0.65 2.40 1.78 0.60

SHUQRA 1.42 0.51 0.23 0.09 2.34 0.58

KUHLAN 0.01 0.01 0.00 0.02 0.00 0.00

Arab J Geosci

Qishn Formation, the basal formation of the TawilahGroup. The succession of the Sa’ar Formation is composedof limestones interbedded with marls, claystones, and shale,where the upper part is dolomitic. The age to the base ofthis formation is 134 Ma. Its initial porosity is 67% and theaverage present porosity is 15%. The present thickness ofSa’ar Formation in the selected wells are 465, 436, 273,380, 450, and 417 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively. The highest average rate ofdeposition of this formation is 4.65 cm/1,000 years. It isobserved in Kharir 1-37 southwest of the study area. Thelowest one is 2.7 cm/1,000 years, obtained from Kharir 1-11 northwest of the study area. The decompacted thickness

due to the sequential removal of compaction effects of thisformation in the studied wells are 946, 898, 609, 804, 922,and 867 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). The highesteffect of compaction on this formation is observed in Kha1-37 well where about 481 m of sediment is compactedunder effect of sedimentary loading above this formationand tectonic subsidence. The tectonic subsidence values ofthe basements during the deposition of this formation are978.9, 867.8, 533.5, 741.9, 990.9, and 725.6 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively(Table 5). The highest tectonic subsidence rate is 990.9 min Kharir 2-20 northeast of the study area.

Table 5 Calculated values of tectonic subsidence of the basement at each well

Formations Kha 1-37 (km) Kha 1-25 (km) Kha 1-11 (km) Kha 1-15 (km) Kha 2-20 (km) Kha 1-16 (km)

KUHLAN 0.004889 0.004178 0 0.006628 0 0

SHUQRA 0.3482 0.139842 0.061873 0.031859 0.518489 0.151071

MADBI 0.592239 0.451568 0.149493 0.363079 0.536246 0.20686

NAIFA 0.592182 0.458341 0.17054 0.32979 0.64147 0.243561

SA’AR 0.978948 0.867834 0.533541 0.741939 0.990935 0.725624

UNCONFORMATY 0.978948 0.867834 0.533541 0.741939 0.990935 0.725624

BIYAD 0.933894 0.848951 0.567462 0.729378 0.959525 0.710299

QISHN 1.005593 0.920579 0.650705 0.798856 1.026606 0.792593

UNCONFORMATY 1.005593 0.920579 0.650705 0.798856 1.026606 0.792593

HARSHIYAT 1.240734 1.206228 0.979286 1.086638 1.300509 1.052395

FARTAQ 1.244492 1.207129 0.976284 1.090535 1.30002 1.060643

MUKALLAH 1.390908 1.360758 1.148753 1.251511 1.436783 1.241129

SHARWAYN 1.401277 1.365535 1.159087 1.263284 1.441413 1.246262

UMM ER RAdHUMAH 1.514566 1.495079 1.281597 1.368601 1.563269 1.357158

Table 6 Rate of total subsidence (cm/1,000 years) of the basement during the deposition of each formation in the selected wells

Formations Kha 1-37(cm/1,000 years)






KUHLAN 0.0136 0.0116 0.0000 0.0184 0.0000 0.0000

SHUQRA 2.9017 1.1654 0.5156 0.2655 4.3207 1.2589

MADBI 7.4030 5.6446 1.8687 4.5385 6.7031 2.5857

NAIFA 11.8436 9.1668 3.4108 6.5958 12.8294 4.8712

SA’AR 19.5790 17.3567 10.6708 14.8388 19.8187 14.5125

UNCONFORMATY 48.9474 43.3917 26.6770 37.0970 49.5467 36.2812

BIYAD 18.6779 16.9790 11.3492 14.5876 19.1905 14.2060

QISHN 5.5866 5.1143 3.6150 4.4381 5.7034 4.4033

UNCONFORMATY 25.1398 23.0145 16.2676 19.9714 25.6652 19.8148

HARSHIYAT 10.3394 10.0519 8.1607 9.0553 10.8376 8.7700

FARTAQ 41.4831 40.2376 32.5428 36.3512 43.3340 35.3548

MUKALLAH 6.0474 5.9163 4.9946 5.4414 6.2469 5.3962

SHARWAYN 28.0255 27.3107 23.1817 25.2657 28.8283 24.9252

UMM ER RAdHUMAH 8.9092 8.7946 7.5388 8.0506 9.1957 7.9833

Arab J Geosci

The total subsidence values of the basement during thedeposition of Sa’ar Formation, calculated using the 1Dbackstripping method are 1,584, 1,349, 756, 1,112, 1,620,and 1,067 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest subsidence rates are observed in Kharir 2-20northeast of the study area.

Biyad Formation

The Biyad Formation rested unconformably above NayfaFormation. It consists of about 69.5% sandstone, 23%shale, and 7.5% limestone. The age to the base of thisformation is 133 Ma. A time span of about a million yearsis represented by a surface of unconformity, erosion, ornon-deposition before the deposition of Biyad Formation iscommenced. The initial porosity of the formation is 53%calculated using Flex-Decomp software and the averagepresent porosity is 28%. The present thickness of BiyadFormation in the selected wells are 298, 275, 248, 319, 318,and 296 m, and the decompacted thickness are 433, 469,365, 401, 466, and 431 m in Kharir 1-37, 1-25, 1-11, 1-15,2-20, and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9).The highest average rate of deposition for this formation is5.31 cm/1,000 years, observed in Kharir 1-15 in the center ofblock 10. The lowest one is 4.1 cm/1,000 years, obtainedfrom Kharir 1-11 northwest of the study area. The highesteffect of compaction on this formation is observed in Kha 1-37 well where about 194 m of sediment is compacted. Thetectonic subsidence values of the basements during thedeposition of Biyad Formation are 933.9, 848.9, 567.5,729.4, 959.5, and 710.3 m, and the total subsidence valuesare 1,690, 1,493, 916, 1,238, 1,751, and 1,202 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively(Tables 5 and 6). The highest tectonic subsidence rateobserved is 959.5 m in Kharir 2-20 northeast of the studyarea.

Qishn Formation

It is the most important formation in the Say’un-MasilahBasin because it is the main oil reservoir. It is depositedconformably overlying the Biyad Formation. The QishnFormation is composed of three members, namely, theSa’af Member, Qishn Clastic Member, and the Upper QishnCarbonate Member. The age to the base of this formation is127 Ma. Its initial porosity is 65% and the average presentporosity is 25.2%. The present thickness of Qishn Forma-tion in the selected wells are 115, 107, 100, 97, 109, and112 m, and the decompacted thickness are 233, 221, 206,199, 223, and 227 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest average rate of deposition for this formation is

0.95 cm/1,000 years, observed in Kharir 1-37. The lowestone is 0.8 cm/1,000 years, obtained from Kharir 1-15 northof the study area. The highest effect of compaction on thisformation is observed in Kha 1-37 well where about 118 mof sediment is compacted. The tectonic subsidence valuesof the basements during the deposition of Qishn Formationare 1,005.6, 920.6, 650.7, 798.8, 1,026.6, and 792.6 m, andthe total subsidence values are 1,821, 1,622, 1,052, 1,359,1,876, and 1,342 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively (Tables 5 and 6). The highestsubsidence rates are observed in Kharir 2-20 northeast ofthe study area.

Harshiyat Formation

It rests unconformably above Qishn Formation. The age tothe base of this formation is 112 Ma. A time span of aboutthree million years is represented by a surface of uncon-formity, erosion, or non-deposition before the deposition ofHarshiyat Formation is commenced. The lithology of thisformation consists of sandstone with interbedded shales andmarls. The initial porosity of the formation is 53% and theaverage present porosity is 34.7%. The present thickness ofHarshiyat Formation are 698, 773, 747, 731, 799, and662 m, and the decompacted thickness are 860, 952, 920,898, 972, and 824 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest average rate of deposition is 5 cm/1,000 years,observed from Kharir 2-20, and the lowest one is4.1 cm/1,000 years, obtained from Kharir 1-16 in thesoutheast. The highest effect of compaction is observed inKha 1-25 well where about 179 m of sediment iscompacted. The tectonic subsidence values of the basementduring the deposition of Harshiyat Formation are 1,240.7,1,206.2, 979.3, 1,086.6, 1,300.5, and 1,052.4 m, and thetotal subsidence values are 2,376, 2,265, 1,725, 1,981,2,516, and 1,907 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20,and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8, and 9). Thehighest tectonic subsidence rate is 1,300.5 m in Kharir 2-20northeast of the study area.

Fartaq Formation

It lies conformably above Harshiyat Formation. It isconsists of 55% shale, 29% limestone, and 16% dolomite.The age to the base of this formation is 96 Ma. Its initialporosity is 66% and the average present porosity is 39.8%.The present thickness of Fartaq Formation in the selectedwells are 34, 35, 25, 35, 33, and 38 m, and thedecompacted thickness are 59, 61, 44, 61, 57, and 67 min Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells,respectively (Figs. 4, 5, 6, 7, 8, and 9). The highest averagerate of deposition is 1.26 cm/1,000 years, observed from

Arab J Geosci

Kharir 1-16, and the lowest one is 0.8 cm/1,000 years,obtained from Kharir 1-11 north of the study area. Thehighest effect of compaction is observed from Kha 1-16well where about 29 m of sediment is compacted. Thetectonic subsidence values of the basement during thedeposition of Fartaq Formation are 1,244.5, 1,207.1, 976.3,1,090.5, 1,300, and 1,060.6 m, and the total subsidencevalues are 2,394, 2,281, 1,732, 2,000, 2,530, and 1,931 min Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells,respectively (Figs. 4, 5, 6, 7, 8, and 9). The highest tectonicsubsidence rate is 1,300 m in Kharir 2-20 northeast of thestudy area.

Mukalla Formation

Mukalla Formation is the largest formation in the MahraGroup. It mainly consisted of sandstone with little siltstoneand shale. The age to the base of this formation is 93 Ma.Its initial porosity is 51% and the average present porosityis 42.5%. The present thickness in the selected wells are526, 534, 520, 532, 503, and 585 m, and the decompactedthickness are 572, 583, 570, 576, 546, and 632 m in Kharir1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively(Figs. 4, 5, 6, 7, 8, and 9). The highest average rate ofdeposition for this formation is 2.25 cm/1,000 years,observed in Kharir 1-16. The tectonic subsidence valuesof the basements during the deposition of MukallaFormation are 1,390.9, 1,360.8, 1,148.8, 1,251.5, 1,436.8,and 1,241.1 m, and the total subsidence value of thebasement are 2,753, 2,651, 2,116, 2,375, 2,869, 2,349 m inKharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells,respectively (Figs. 4, 5, 6, 7, 8, and 9) and (Tables 5 and 6).The highest tectonic subsidence rate is 1,436.8 m in Kharir2-20 northeast of the study area.

Sharwayn Formation

It is deposited conformably overlying the Mukalla Forma-tion. It consists of 71% limestone and 21% dolomite. Theage to the base of this formation is 67 Ma. Its initialporosity is 64% and the average present porosity is 55%.The present thickness of Sharwayn Formation are 33, 19,30, 35, 18, and 20 m, and the decompacted thickness are41, 24, 38, 43, 22, and 25 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively (Figs. 4, 5, 6, 7, 8,and 9). The highest average rate of deposition is0.58 cm/1,000 years, observed in Kharir 1-15. The lowestone is 0.3 cm/1,000 years, obtained from Kharir 1-11 northof the study area. The tectonic subsidence values of thebasements during the deposition of Sharwayn Formationare 1,401.3, 1,365.5, 1,159, 1,263.3, 1,441.4, and1,246.3 m, and the total subsidence values are 2,773,2,662, 2,136, 2,398, 2,879, and 2,360 m in Kharir 1-37, 1-

25, 1-11, 1-15, 2-20, and 1-16 wells, respectively (Figs. 4,5, 6, 7, 8, and 9 and Tables 5 and 6). The highest tectonicsubsidence rate is 1441.4 m, observed in Kharir 2-20northeast of the study area.

Umm Er Radhuma Formation

It is the last formation in the stratigraphic section of block10. It represents the first formation of Hadramawt Group. Itis deposited conformably above Sharwayn Formation andcomposed mainly of limestone. The age to the base of thisformation is 61 Ma. The initial porosity of the formation is70% and the average present porosity is 63.5%.

The present thickness in the selected wells are 260, 292,293, 245,269, and 256 m in Kharir 1-37, 1-25, 1-11, 1-15,2-20, and 1-16 wells, respectively. The highest average rateof deposition for this formation is 1.7 cm/1,000 years,observed in Kharir 1-25 and 1-11 wells, and the lowest oneis 1.4 cm/1,000 years in Kharir 1-15 in the central of theblock. There is no thickness correction made for thisformation due to the compaction. The tectonic subsidencevalues of the basements during the deposition of Umm ErRadhuma Formation are 1,514.6, 1,495, 1,281.6, 1,368.6,1,563.3, and 1,357.2 m and the total subsidence values are2,953, 2,868, 2,335, 2,565, 3,070, and 2,536 m in Kharir 1-37, 1-25, 1-11, 1-15, 2-20, and 1-16 wells, respectively(Figs. 4, 5, 6, 7, 8, and 9 and Tables 5 and 6). The highesttectonic subsidence rate is 1563.3 m observed from Kharir2-20 northeast of the study area.

Conclusion

Geohistory Analysis is carried out on block 10 in theSay’un-Masilah Basin. The present study is based on theanalysis of the well logs of six exploration wells.

Backstripped subsidence curves, constructed by remov-ing the effects of processes such as sediment loading,loading due to the water column, and sediment compaction,allow the subsidence histories of the basin to be examinedin terms of the tectonic drivers that caused the subsidenceof the basins.

The rates of sediment accumulation for the studiedformations are shown in Table 4. The highest accumulationrates are 9.01, 6.66, and 6.37 cm/1,000 years for the Sa’arFormation, Harshiyat Formation, and Biyad Formation,respectively.

The rates of the total subsidence of the basement duringall stages are shown in Table 6. From the Middle Jurassic tothe Eocene, the Say’un-Masilah Basin exhibits a complexsubsidence history over a period of about 100 Ma. Thehighest rate of tectonic subsidence commenced at 150 Maassociated with the extensional rifting of the basin.

Arab J Geosci

In the Early to Middle Jurassic, slow subsidence wasdriven under the effect of sedimentary weight, whenKuhlan and Shuqra Formations were deposited and formedthe pre-rift sediments. After that, rapid subsidence happenedin the Middle to Late Jurassic that was driven by mechanicalextension and rifting at this time, forming the Syn-riftsediments, represented by Madbi Formation.

In the Middle Jurassic, the dynamic loading was over-whelmed by static loading, caused by the developing hugeaccumulative sediments, leading to very high rates ofsubsidence, and the development of Say’un-Masilah Basin.

The mechanical extension ceased at about 144 Ma, beingreplaced by a phase of post-rift subsidence, resulting inmore widespread uniform sedimentation, with reducedtectonic subsidence rates forming the post-rift sedimentsrepresented by Nayfa, Sa’ar, and Biyad formations and allMahra Group formations. This stage was followed by ahigh rate of sedimentation during the Late Paleocene toEarly Eocene, forming the Um Er Radhuma Formation witha high rate of subsidence. This rate of tectonic subsidencecould be interpreted in terms of dynamic re-rifting duringthis time.

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