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Maturation of a New Play Concept in Northern Provinces of Offshore Sarawak Basin, Malaysia

S.R. Iyer* (PETRONAS Exploration), H. Rosidah (PETRONAS Exploration) & A. Shahrul Amar (PETRONAS Exploration)

Northern provinces of Offshore Sarawak Basin represent a frontier area with low well density and relatively fair understanding of the petroleum system. Hydrocarbon prospectivity evaluationintegrating seismic, gravity, and well data of northern provinces of offshore Sarawak facilitatedmodeling of tectono-stratigraphic framework and geological evolution of the area, culminating in thedevelopment of a new play concept, hitherto untested in this part of Sarawak Basin. Stratigraphy ofthe study area is characterized by a lower synrift sequence, consisting of transitional to shallow marine sediments, unconformably overlain by passive margin sag phase deep water sediments.Exploration to date has focused on hydrocarbon plays in upper part of synrift section. Present paperdeals with maturation of Sequence-A play as a new concept, primarily based on seismic evidence.This sequence, consisting of postulated lacustrine sediments, filled the early formed half grabensduring the first phase of extension. Uplifted sections of this sequence on footwall closures arepotential hydrocarbon plays with charge from adjoining grabens. Regional review of equivalentsections suggests clastic sequence similar to Sampaguita-1 as the most likely analogue. Proving this play will open up substantial exploration potential in the northern provinces of Offshore Sarawak Basin, with significant economic value.

Introduction

Offshore Sarawak Basin, located towards south western part of South China Sea, is an area of activehydrocarbon exploration and development for more than a century. The present paper covers Northand West Luconia Provinces of Offshore Sarawak Basin, where active exploration commenced from1990. The study area (Fig.1) is a frontier basin with low well density, and poor to fair understanding of the petroleum system. Regional hydrocarbon prospectivity study integrating all geoscientific datafacilitated modeling of tectono-stratigraphic framework and geological evolution of the area,culminating in the development of a new play concept, hitherto untested in this part of SarawakBasin.

Geological Background

General stratigraphy of the study area is shown in Fig.2. Northern provinces of Offshore SarawakBasin initiated as intracratonic rifts on attenuated continental crust, on the foreland bulge part, duringLate Cretaceous (?) to Late Eocene. The general structural style during this stage is characterized byN-S and NE- SW trending half grabens, dipping to the east and southeast (Fig.3). Sequence-A (Pre Cycle-I sequence of earlier authors), consisting of possible lacustrine facies (?), filled the earlyformed half grabens. The extensional phase continued, with opening of South China Sea during EarlyOligocene, and also during subsequent drift phase up to Early Mid Miocene. Unequal subsidence inthe half grabens over the study area during this stage, accommodated variable thickness of infillsequences B and C, represented by Cycles I to III (Fig.4), showing diverse facies distribution as evidenced by the wells. It can be inferred that the study area consisted of a complex of minibasins/grabens with uneven subsidence history, bounded by intervening horsts, often forminglocalized provenances. Subsequent regional uplift in the area associated with plate convergence during Late Early Miocene to Middle Miocene, resulted in a major unconformity, designated MiddleMiocene Unconformity (MMU). Post unconformity easterly and northeasterly sag of the basinthrough Late Miocene to Recent lead to a deep water setting, ensuing in deposition of hemipelagics,overlain by mass transport dominated sequence.

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Sequence-A Play Concept

Exploration to date has focused on hydrocarbon plays in sequences B and C, wherein, a few subcommercial HC accumulations have been discovered. Sequence-A, which constitute the pre rift/early synrift section, is quite evident in the 3D seismic data of the area (Fig.5), and shows unequivocalsedimentary characters like layering, angular unconformity etc. Multiple units can be mapped withinthis sequence, showing distinct seismic characters representing discrete facies. The postulated lacustrine clastic sediment fill includes the source, reservoir, and intraformational seal elements ofSequence-A petroleum system. Regional top seal for this play is transgressive shale (?) overlying theSequence-A unconformity. Uplifted sections of this sequence on footwall closures are idealexploration targets for this play. Prospect-A is an example of Sequence-A play with three way footwall closure against a fault forming a structural trap (Fig.5). The predicted depth to the target isless than 2000m below mudline and reasonably good porosity is expected based on offset wellporosity-depth plots. Reservoir temperature predicted is less than 1250C and any oil accumulated is likely to be preserved without cracking.

Regional screening of equivalent sections for analogues has brought out Sampaguita-1 well in Reed Bank Basin towards NE as the most likely, suggesting a clastic play in Sequence-A (Fig.6). Transitional to non marine sand/shale sequence of Early to Mid Cretaceous age, up to 600m thick, was reportedly penetrated in equivalent section in Sampaguita-1. Two shallow offshore wells in the SW part of Sarawak drilled in 1970’s encountered tight, feldspathic sandstone of Eocene (?) age,unconformably overlain by Cycle V section, and possibly represents Sequence-A equivalent in the southern part of Sarawak, around 600 km from the study area. Several seismic profiles indicate likelycarbonate reflection character in Sequence-A highs, similar to a Pre Tertiary hydrocarbon discoveryin Vietnam (Fig.7). Consequently, carbonate reservoir in Sequence-A is a likely scenario. Metasediments varying in age from Late Cretaceous to Eocene constitute the economic basement inseveral deep wells in East Natuna basin, fringing the western boundary of the study area, making up a possible case. True basement represented by granodiorite has been observed in only one well in EastNatuna, which is an unlikely scenario for Sequence-A, based on seismic character.

Proving this play will open up substantial exploration potential in the northern provinces of OffshoreSarawak Basin, where, a significant number of potential leads have been identified, similar to theprospects shown in Fig.4, with significant commercial value.

C - Sequence-A play is a new concept targeting Pre/Early Tertiary sequence, hitherto untested in the

northern provinces of Offshore Sarawak Basin. - Proving this play will open up substantial exploration potential in the study area, with significant

commercial value.

onclusions

References 1. Andrew Cullen, Paul Reemst, Gijs Henstra, Simon Gozzard and Anandaroop Ray, [2010]. Riftingof South China Sea: new perspectives. Petroleum Geoscience, Vol. 16 2010, pp 273-282. 2. Kenneth Thies, Mansor Ahmed, Hamdan Mohamad, Richard Bischke, Jeffrey Byer and DanielTearpock, [2006]. Structural and Stratigraphic Development of Extensional Basins: A Case StudyOffshore Deepwater Sarawak and Northwest Sabah, Malaysia. Search and Discovery Article #10103 (2006). 3. Mazlan H Madon, Kim C Ly, Robert Wong, [2007]. The Structure and Stratigraphy of Deep waterSarawak, Malaysia. Implications for the Tectonic Evolution of NW Borneo continental margin.AAPG Annual Convention and Exhibition, 2007, Long Beach, CA.

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Delivering Value: Realising Exploration & Development Potential Kuala Lumpur, Malaysia, 23‐24 April 2012 

 

 

 

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Kuala Lumpur, Malaysia, 23‐24 April 2012  

 

 

Petroleum Geoscience Conference & Exhibition  

Delivering Value: Realising Exploration & Development Potential Kuala Lumpur, Malaysia, 23‐24 April 2012