Onshore to Offshore Trends in Carbonate Reservoir Quality across a Land-Attached Shelf in SE Asia

Wilson, Moyra E.¹; Chang, Eva¹; Lunt, Peter²; Welsh, Kevin³
¹Applied Geology, Curtin University, Perth, WA, Australia.
²Mitra Energy, Kuala Lumpur, Malaysia.
³University of Queensland, Brisbane, VIC, Australia.

Although isolated Miocene buildups in SE Asia commonly form prolific hydrocarbon reservoirs, their equivalents on clastic-dominated land-attached shelves remain poorly known and underexplored. Here, onshore to offshore trends in carbonate development and reservoir quality are assessed across the N Borneo shelf through study of surface outcrops and subsurface wells. A multidisciplinary programme of fieldwork, petrography and geochemical analyses allowed evaluation of spatio-temporal variations in, and controls on, deposition, diagenesis and porosity. In addition to field logging and sample collection >200 samples were studied via standard, cathodoluminescent and scanning electron microscopy together with isotopic (O, C & Sr) plus trace/major elemental characterisation.

Carbonates developed as localised low, and higher relief buildups, as well as more continuous sheet like deposits in near-coast to shelf margin positions. Molluscs, corals, larger benthic foraminifera and coralline algae are common constituents. Most samples show evidence for marine, but predominantly burial, diagenesis. Near-coastal carbonates commonly contain siliciclastics. Some early, probable meteoric leaching affected inner shelf deposits prior to pervasive neomorphic to blocky/poikilotopic calcite cement formation. Cementation probably reflects onshore carbonate leaching and then downdip reprecipitation in meteoric aquifers derived from the humid landmass of Borneo. Despite this cementation, higher energy inner shelf grainstones still retain, or have enhanced porosity over their lower energy counterparts.

Late burial dissolution (associated with saddle dolomites) enhancing predominantly primary, and minor secondary porosity is key to reservoir quality development. Although some late leaching occurs across the shelf, best porosity remains in high energy grainstones and rudstones in platform edge / outer shelf positions isolated from clastic influx. Platform margin areas are also the most favourable sites for burial corrosion if leaching fluids are generated in adjacent basins (e.g. compaction and catagenesis of organics). Critical factors in reservoir quality development common to SE Asian shelves are: (1) burial leaching and fluid pathways, (2) development and preservation of primary porosity, and (3) cementation associated with meteoric aquifers draining large humid equatorial landmasses.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012