ABSTRACT: Sedimentological/Petrological Analysis of Reservoir Units within the Fluvial/Estuarine/Marine Depositional Complex of the Talang Akar Formation (Oligocene), Bentayan Field, South Sumatra, Indonesia

SIEMERS, CHARLES T., Consultant, P.T. Geoservices, Jakarta, Indonesia, and RICHARD A. LORENTZ, Asamera Oil, Ltd., Jakarta, Indonesia

The Bantayan field, located on the northeast flank of the South Sumatra basin, was delineated by surface geological studies in the late 1920s as a narrow northwest trending anticlinal structure bisected by numerous northeast trending transbasinal normal faults. A 1932 discovery well encountered productive upper Talang Akar sandstones which yielded 800 BOPD of paraffinic based crude with a pore point of 115 degrees F. Only 12 delineation wells were drilled over the next 50 yr; however, data from those wells and two periods of seismic reflection surveys revealed up to 12 potentially productive sandstone units within a structural closure of 27 sq km. A 1989 8-well, oriented-coring program provided an abundance of additional material on which this study was based. An extensive field drill ng program, guided by the results of this study, is now underway and has demonstrated the importance of detailed subsurface stratigraphic/sedimentological models for the development of such complex reservoir systems.

The detailed sedimentological/petrological examination of over 225 m of Talang Akar core from 8 wells (150 m of which was oriented) has delineated at least six major depositional sequences with highly variable reservoir quality. From the top downward they are: (1) Upper Transgressive Marine (7-21 m), (2) Upper Stacked Fluvial Channels (5-6 m), (3) Prograding Splay Delta Complex with Platform Coals (12-15 m), (4) Middle Stacked Fluvial Channel Braidplain (9-16 m), (5) Tidally-Influenced Estuarine/Marine Deposits (10-30 m), and (6) Lower Stacked Fluvial Channel Braidplain (7-25+ m). These sequences are laterally continuous throughout the study area but all sequences contain laterally discontinuous sedimentary bodies.

All depositional sequences contain sandstone bodies; however, the stacked fluvial channel braidplain deposits are the only ones with favorable reservoir potential. Sandstone bodies within such sequences commonly display erosional bases, fining-upward trough cross-bedded sandy pebble conglomerates to fine-grained sandstones and upper abandoned channel-fill. Considerable sandbody variation can be demonstrated laterally and vertically; however, for the most part, they tend to merge into an overall well-connected braidplain type reservoir system. Sandstones range from quartz arenites to sublitharenties with variable amounts of metamorphic rock fragments and feldspar. Total measured porosity and permeability of the most favorable reservoir units range 20-25% and hundreds of units range to ,000+ md. The distribution of porosity within such units includes: (1) remnant primary intergranular macroporosity (10-15%), (2) secondary grain-dissolution macroporosity (4-7%), and (3) microporosity (4-6%, mostly within authigenic clays). Reservoir sandstones contain an average of 10% cement, including mainly silica (4-6% quartz overgrowths) and pore-filling, authigenic kaolinite (3-5%).

In the Bentayan Field, Talang Akar depositional complex, the Middle Stacked Fluvial Channel Braidplain sequence displays the best overall reservoir quality, for the following reasons: (1) development of relatively thick and well-connected, fine- to coarse-grained, cross-bedded fluvial sandstones, (2) relatively minor detrital mud matrix, (3) mineralogically mature framework fraction, except for moderate amounts of feldspar, (4) only moderate mechanical compaction with resulting preservation of much of the primary intergranular macroporosity, and (5) development of secondary porosity owing to feldspar dissolution. The only major reservoir degrading element is the occurrence of moderate amounts of authigenic grain-coating and pore-filling clay minerals which not only reduce macroporosit but can break away and migrate during hydrocarbon production, resulting in clogged pore throats.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)