Abstract
The outcrop of shallow marine sandstone, Belait formation at Meragang Beach, Brunei Darussalam have long been regarded as tide-, wave- and storm-dominated environment that reveals the reservoir heterogeneities and variation in sand body geometry. Intensive outcrop study methods were available from previously conducted work on Belait Formation. But for this study the priority was to acquire gamma-ray profile at 5-10 cm scale by using Digital Total GR Count Detector and re-logged of Sedimentological log (in cm) of 311.8 m thick outcrop to describe the depositional environment and sedimentary facies architecture. The sediments deposited in a tide-dominated environment mostly interpreted in all cycles which were characterized by sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment with sedimentary facies of planner cross-bedding, trough cross-bedding, herringbone cross-stratification, wave ripples and thin parallel lamination with coal fragmented bed and carbonaceous material. The gamma-ray profile for tide dominated environment shows different shapes; block left-boxcar, bell shape, funnel shape and irregular trend. In wave- and storm-dominated are wave ripples, thin parallel lamination with mud drapes, large planner cross-bedding, trough cross-bedding, highly bioturbated (skalithos) sandstone and hummocky cross-stratification. Whereas, the gamma-ray profile shows different shapes; left-boxcar, right-boxcar, funnel shape and irregular trend and in storm-dominated it show only left-boxcar with no sharp edges on top and bottom. This approach combines excellent data quality to interpretate different depositional environment and its impact on sandstone quality which implies that the outcrop gamma-ray log can use to characterize the rock deposited in different depositional environment in different facies. It also reveals that the changes in gamma-ray profile with in the sandstone depend upon the environment in which the sandbody deposited that reveal in reservoir heterogeneities in subsurface well correlation.
Highlights
The gamma ray and its change in trend in different depositional environment with respect to facies distribution are useful for the characterization of reservoir analogues and to improve the understanding of subsurface well log correlation
We describe the studied outcrop at Meragang Beach by detailed description of the facies, documenting all the characteristics of its lithology, texture, sedimentary structures and traces fossil which can aid in determining characterized by different sub-depositional environment which includes; tidal channel, muddy tidal flat and mixed tidal flat sub-environment
The GAMMA RAY (GR) log is commonly used by the geologist to distinguish different sedimentary environments as the fining and coarsening upward cycles are often related to changes in grain-size and clay contents (Cant, 1992)
Summary
The gamma ray and its change in trend in different depositional environment with respect to facies distribution are useful for the characterization of reservoir analogues and to improve the understanding of subsurface well log correlation. Outcrop gamma-ray logging and facies description is an applicable tool that can provide invaluable information on the thickness of sand body, geometry, grain size, lithology, shale sand cut-outs and depositional environment. They are most useful when combined with core or image log data, biostratigraphic information and other well log (Rider, 1996). This study documents the facies distribution and outcrop gamma-ray log profile in shallow marine tide-, wave- and storm-dominated depositional environment sandstone of South Brunei, Muara District along. Where supported mud is high the sand is well-sorted figured in exposed shale section) was applied onto the and fine- to medium-grained
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