case study salt dome exploration

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Case study: Seismic acquisition research on salt dome structure in Tarim BasinXu Ligui*, Yan Feng and Shi Haifeng , BGP , CNPC

Summary

Salt dome structure is an important object in oil explorationin Tarim basin. In the year 2002, the goal of the seismicacquisition research in Quele-zhongka is to locate the saltdome structure of the Tertiary in the area. We implemented

the delicate near-surface survey and statics, optimized thespecific 2D layout for the structure of deep salt domes, andintensified the excitation in single hole deep to highvelocity layers in mountain areas. As a result, the quality of

the seismic section has been obviously improved, and thestructures of salt domes and anticlines are clearly displayed

in the section.

Introduction

Tarim basin, located in the west of China with an area of560,000km2, is the biggest oil-gas-bearing basin in China .

In the significant thickness of sedimentary mantle formedin Camberian, Carboniferous, and Tertiary periods, thereexit four layers of salt beds, among which the salt beddistributed in the lower Tertiary system of Kuche forelandbasin in the north of Tarim is related most to the

enrichment of petroleum and natural gas. In the light of thepractice in many years, it is possible to find out the oil-gas-reservoirs so long as the structural configuration of salt

dome, especially the sub-salt structural configuration andtrap is clear. Affected by the features of salt dome itself andthe complex surface and underground geological structure

in the foreland thrust fault belt, the data quality of some saltdomes in Kuche foreland basin is very poor, even noreflection occurs somewhere. This badly restricts the

further discovery of oil-gas reservoirs. To resolve thisproblem, a research project, 2-D seismic acquisition on saltdomes, was carried out by BGP and Tarim Oil fieldCompany, CNPC in 2002.

Difficulties in exploration

Research area, Quele-Zhongka district, is located in thesouthwest part of Kuche foreland thrust fault zone, thenorth of Tarim basin (Fig.1). The thick salt beds developedin the lower Tertiary system were strongly pressed during

the later period of Himalayan orogeny, and plasticallydeformed into many salt domes, which are usually buriedbelow the depth of 4000m. The thick sandstone layerbeneath the salt domes is the main reservoir of petroleum

and natural gas. So it would not be possible to find out thesub-salt reservores unless the structures of salt domes were

found out. The obstacles display at four aspects as follows:

x The conditions of seismic acquisition operations arevery poor because of complex and various types of

topographies. In this area, there are many types oftopographies such as the mountains, the Gobi desert,incompact soil covering layers, and swamps and so on.In mountain areas, the surface is badly cut by gullies

developed in different directions. The maximum relativeelevation difference in the mountain area may reach ashigh as 200m. Such complex topographies result in theextreme difficulties and very low efficiency in theseismic acquisition operations.

x Static corrections are very hard to do owing to greatdifferences in the near-surface structures. The thickness

and velocities of low-velocity layers in the area varyrapidly both in lateral and vertical directions. Inmountain areas, the structure of the low-velocity layersis not clear, and their thickness may reach 100m; in

swamp areas, the layer has the structure of double sub-layers, weathering thickness is between 2 and 10m; inGobi area, the incompact soil covering layer is also with

double sub-layer structure, and the thickness of the layeris 8 to 50m. All of these make the near-surface surveysand precise statics very difficult.

x Receiving and shooting condition are poor and theabsorption and attenuation of seismic waves are strongin mountain areas. The mountain bodies in this area

were formed during the later period of Himalayanorogeny in the end of Quaternary period. The width of

the mountain areas along measuring lines are generally5000~8000m. In the near-surface of the mountain bodies,not only the thick low-velocity layers are developed, but

also exist structural cracks and rain eroded holes. So theattenuation of seismic waves in acquisition profiles isvery strong, and may further badly influence theeffective reflecting information of salt domes and the

sub-salt structures.

x The imaging quality of salt domes and the sub-saltstructures is very bad due to the large embedding depthof salt beds and serious breakage of the overburden of

Fig.1: Schematic diagram for the location of the seismic

acquisition research area

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Case study: Seismic acquisition research on salt dome structure in Tarim Basin

the salt beds. In the area, the embedding depth of saltbeds is about 4000~5500m, and the depth of petroleum

and natural gas is 5000~6000m. The main part of thesalt dome structure is just in the position of the mountainareas. Because the action of push-up tensile stresscaused by the salt dome, a large amount of cracks andfaults in the overburden of the salt dome are produced.

This will further result in the strong absorption ofreflecting seismic wave energy so that the ratio ofsignal-to-noise of the salt dome and the sub-salt strata

becomes very low and their imaging effect is very poor.

Acquisition techniques and strategies

The aim of the seismic acquisition research in 2002 is to

obtain seismic data with good reflection from the deep saltdome structure. Considering the difficulties in explorationin this area, firstly, we try to solve the static corrections byimplementing delicate near-surface structure survey,

establishing the accurate near-surface velocity model anddoing fine statics computation; secondly, we optimize thegeometry design to be suitable for the receiving of the

reflecting waves both from deep salt dome and fromshallow layers with high S/N; thirdly, we increase theshooting energy by drilling single shot hole deep into highvelocity layers in mountain areas where there exist verythick covering layers, so the quality of the raw seismic datagets improved. The techniques applied in the seismic

acquisition research are as follows:

1. Delicate near-surface survey and statics

Comprehensive near-surface surveys

Flexible refraction survey methods are applied according todifferent near-surface conditions. We apply the spreadlength of 198m in refraction survey in swamp or Gobi

desert area where the relatively thin weathered layers arecovered, but in the refraction exploration of the Gobi desert

area with relatively thick weathered layers in front of themountain, two spread lengths of 656m and 198m areadopted. The former is for refraction survey, the latter is fortracing survey.

The depth and the distribution points of uphole surveys areenlarged in mountain areas. Within the strip of mountain

bodies by the size of 5500m long and 400m wide, 13uphole surveys are all drilled to the depth of 80m so thatthe high velocity layers are reached in every uphole survey.

Detailed outcrop survey is carried out. Lithology andstratigraphic dip survey is performed and the outcropprofiles along the exploration line are plotted so that thebasic data can be obtained by selecting the proper

acquisition parameters and establishing the near-surfacemodel.

Tomography exploration on the near-surface is practiced.The near-surface velocity model is inversed by the Fathom

software so as to get a higher inversion accuracy.

Comprehensive near-surface structure model building.

According to the characteristics of the near-surfacestructures, the different near-surface structure models are

established by different methods. In swamp, Gobi anddesert areas, the linear interpolation method is applied tointerpolate the velocity field of the low-velocity layers and

top interface of the high-velocity layer, while in mountainareas, besides what is applied above, the similaritycoefficient method is also practiced to interpolate thethickness of the weathered layer.

Optimization of static correction methodThe quantities of the statics are calculated with four kindsof methods, i.e., elevation method, first break refraction-based method, zonation based model-base method and

tomographic method. The results of different staticscorrection methods are compared in shot gathers, receivergathers and stacked sections to find out the most suitable

method for the area. It is found that the continuity andsmoothness of the first arrival in gathers resulted frommodel-based and tomographic methods are better than thatfrom other two methods, and the best is from tomographicmethod (Fig.2).

2. Design and application aimed at the mid-depth

and deeper layers

On the base of the geological models set up in the light ofthe legacy data, the careful analysis of the issue about howto observe the reflecting waves from the salt domes is

conducted. It is decided that the acquisition softwareKLSeis2.0, which is developed by BGP, is adopted, and the

Fig.2: Effect of statics in CMP gathers (a: Original. b:

first break refraction-based method. c: model-based

method. d: tomographic method)

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layout should be small group interval, multiple channels,long spread and suitable folds. The parameters of the layout

are as follows:

x group interval: 20m

x Shotpoint spacing: 40/80m

x receiver number: 592

x Layout: 6010-110-20-110,6010

x folds: 74

x Array pattern: 3 string * 10 geophones,areal array withtriangle pattern ,a string of geophone is stretched inlateral direction.

x Array length: Lx = 9m, Ly = 36m

x Geophone separation:x = 3m,y = 4m

On the two flanks of a structure, the layout is changed into4490-110-20-110-7530 manner, and 30 additional shots areadded on each of the flank so that the folds in structural

targets are enhanced to a number over 120.

The layout has some advantages as follows: Small groupinterval can lead to small CMP interval and effective folds

at the top of the salt dome and the shallow layers; The longspread is helpful to the receiving of reflection signals fromdeep layers; In data processing, the noise along the line canbe reduced by trace arrays, and the folds can be enhanced

by the arrays of the CMP gathers. All of these measures caneffectively improve the S/N of the data from shallow layers,the top of the salt domes and the deep layers.

3. Techniques of shot hole deep to high velocitylayers in mountain areas

In research area, the salt domes all exit under the compexmountain areas where the thickness of the weathered layersis significant. So, the past seismic acquisitions were all

implemented with the array of shallow shot holes withinthe weathered layers. Because the S/N of the single shot ina shallow shot hole is very low, reflection signals are

disordered or even there is no reflection at all in the pastseismic sections. During the acquisition research at thistime, a large amount of shot experiments are carried out inthe mountain areas, and the results show that the reflectionsignals from shallow, middle and deep strata are very clearin the records resulted from a single shot record in a shot

hole deep into the high-velocity layers, and the reflectionsignals from the bottom of salt domes and the sub-saltstrata are intensified obviously, their S/N is much higherthan that within the records resulted from the array of shot

holes in the low-velocity layers; and the quality of therecords from a single shot in the depth near the interfacebetween low-velocity layer and high-velocity layer is alsobetter than that in the record from a shot totally within the

low-velocity layers. Therefore, the exciting in a single shothole deep into high velocity layers is an effective approachto improve the S/N of the raw seismic data and the quality

of reflection from salt dome structures. In order to realizethe shooting totally in high velocity layers in such a

mountain areas where the thickness of the low-velocitylayers is near to 100m, four kinds of measures in ouracquisition research are adopted:1) Change the circulation manner, provide proper mud,

and enhance the capacity of seismic drilling rig from

drilling depth of 30m to more than 80m, which breaksthrough the past record level of seismic drilling depthin the mountain areas of Tarim basin.

2) Increase the construction efforts to build the roadalong the research line across the mountains so thatthe drilling machine can be carried to the right placesfor drilling on the mountains.

3) Select the proper place where the shot hole can be

drilled into high-velocity layers. However, thedisplacement of the shot point should be within areasonable range.

4) Use the appropriate explosive with high density to

guarantee the effective coupling between theexplosive and the surrounding rock in the shot hole,and to improve the effect of explosions as well as to

reduce the length of explosive bar and the depth ofshot hole.

Disscussion about the seismic sections

It is the application of the techniques such as delicate near-

surface surveys, statics, the layout aimed at middle anddeep layers, and the single shot holes deep into high-

velocity layers, in addition to strict field operationmonitoring and quality control management, that makes asuccessful seismic acquisition research, through which the

quality of the seismic sections get improved materially.

Fig.3 shows a comparison between the stacked section of

the research line 02A along S-N direction processed at siteand the legacy final stacked section neighboring andparallel to the research line 02A. Although the legacystacked section is the final processing result, the S/N inshallow, middle and deep part of the section is very low,and the outlines of the salt domes and the sub-salt

structures are quite obscure. In the new section, however,the overall quality has remarkably been improved, theoutlines of salt domes and the reflection of the sub-salt

strata are quite clear, and the sub-salt anticline structuresare displayed obviously. The reflections from the top of thesalt domes and the overburden of the salt domes have ahigher S/N and the attitude of the strata is legible and

reliable.

Fig.4 shows the comparison between the stacked section ofthe research line 02B in SW-NE direction processed at site

and the legacy final section intersected with the former one.It can be seen from Fig.4 that both the S/N and the

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resolution of the new section have been greatly improved.The reliable reflection images show up clearly in the places

of the section where no reflection exists in the legacysection. The outlines of the salt domes and the features ofthe structure are clear. The sub-salt structures get furtheraffirmed in the new section.

Conclusions

To get high quality reflection data from the salt domes

developed in lower Tertiary system is the object of ourseismic acquisition. In order to realize this object, wefollow the strategy from the outside to the inside, from

the point to the line. The research starts from the statics ofcomplex near-surface, and next is the improvement ofexcited conditions and the optimization of observinglayouts. The techniques and measures practiced in theresearch are suitable for the research area so that the quality

of seismic sections from the research acquisition getsremarkable improved.

To guarantee the application of the new acquisitiontechniques and strategies, adequate investment, advancedequipment and strict measures of quality control are alwaysnecessary.

The configurations of the salt domes and the sub-saltstructures are clear and reliable. The seismic sections revealthat the scale of the salt domes and the anticlines is quite

large, so the further work such as detailed data processingand exploration is worthy. 3D seismic exploration shouldbe the best choice to locate the salt dome and the sub-salt

structure exactly.

Reference

Li Qingzhong, 1993, Road Toward Precise SeismicExploration, Petroleum Industry Press.

Lu Jimeng, 1993, Principle of Seismic Exploration,Petroleum University Press.

Yan Shixin et., 2000, Mountainous GeophysicalExploration Technology in Mountain Areas, Petroleum

Industry Press.

Acknowledgments

We would like to thank Mr. Yang Juyong, Mr. WanWeihua , Mr. Sun Jinzhong, Mr. Qian Yuping and Mr. LiuXinwen for their great contribution to this paper.

astacked section of research line 02A processed at site

(b) The final stacked section of the legacy section parallel to

line 02A

Fig.3: Comparison between the stacked section of research

line 02A and the legacy section

(a) Stacked section of research line 02B processed at site

(b) The final stacking section of the legacy section intersected with

line 02B

Fig.4: Comparison between the stacking section of research line02B and the legacy section

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case study salt dome exploration