Depositional controls of reservoir thickness and quality distribution in Upper Miocene shallow marine sandstones (Stage IVD) of the Erb West Field, Offshore Sabah, NW Borneo


Author : H.D. Johnson, S. Levell & A.H. MohamadPublication : Bulletin of the Geological Society of MalaysiaVolume : 21Page : 195-230Year : 1987


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Bulletin of the Geological Society of Malaysia, Volume 21, Dec. 1987, pp. 195 – 230

 

Depositional controls of reservoir thickness and quality distribution in Upper Miocene shallow marine sandstones (Stage IVD) of the Erb West Field, Offshore Sabah, NW Borneo

H. D. JOHNSON, S. LEVELL AND A. H. MOHAMAD

Sabah Shell Petroleum Company Limited, Lutong, Sarawak

 

Abstract: The Erb West field, situated offshore west Sabah, is a prominent NE-SW trending anticlinal structure containing substantial oil and gas reserves. The hydrocarbons are contained within a ca. 800 ft thick sequence of shallow marine sandstones and shales, belonging to the Upper Miocene, Stage IVD. The prospective reservoir interval is characterized by marked variations in thickness and rock quality across the field, which significantly influence hydrocarbon distribution and field development strategy; the nature and origin of these variations are discussed in terms of the depositional and tectonic setting of the Erb West field.

Facies analysis of ca. 1200 ft of core (from three wells) indicates that the reservoir sequence comprises five main facies with distinctive textures, sedimentary structures and porosity/permeability characteristics: (1) bioturbated mudstones (12%, 5 mD), (2) bioturbated sandstones (15%, 20 mD), (3) heterolithic sandstones (24%, 150 mD), (4) parallel laminated (hummocky cross-stratified) sandstones (25%, 350 mD) and (5) massive sandstone (26%, 2200 mD). The same facies can also be identified in uncored wells (total = 20) using a combination of GR, FDC, CNL and LLD log cut-off values, together with diagnostic HDT microresistivity curve shapes. Synthesis of these data, supported by regional geological and seismic data, has led to the identification of depositionally-related trends in sand thickness, facies and porosity/ permeability.

Neritic microfaunas and abundant bioturbation indicate a shallow marine or shelf environment of deposition. The sandstones typically display parallel to wavy lamination (hummocky cross-stratification), wave ripples and grading, which are considered indicative of a storm-dominated depositional regime. This storm-dominated sand sequence onlaps against the Shallow Regional Unconformity and is believed to have been sourced by coastal erosion of the nearby, emergent and tectonically-active Erb High. Facies trends within the individual reservoir units support both depositional onlap and the conclusion that the Erb West reservoirs represent a transgressive sand cpmplex.

Thickness and rock quality variations within the field reflect the interplay of depositional trends and subsidence patterns (caused by tectonics and/or relative sea-level changes). In the main N Sands reservoir the following depositional pattern is evident: (1) relatively condensed, "proximal" sequences to the E and SE, (2) well-developed, complete ("intermediate") sequences, with coarsening upward sand bodies, in the centre of the field, and (3) well-developed but finer grained, "distal" sequences to the W and NW, which includes a shale-out on the western flank. Superimposed on this facies trend is a subsidence trend which is manifested by a northward increase in the gross thickness of individual reservoir units (i.e. increasing towards the more distal part of the basin); the southward reduction in thickness reflects onlap against the Shallow Regional Unconformity. The resulting reservoir quality maps show optimum sand development in the central part of the field, seen as NE-SW trending sand "thicks", which decrease in both proximal (to E and SE) and distal (to W and NW) directions.

This reservoir geological model provides a basis for the following applications: (1) reserves estimates, (2) reservoir simulation studies, (3) monitoring well performance, (4) optimizing development well locations, and (5) further development planning.