Reservoir rock properties of the Arun limestone (Miocene), Arun field, North Sumatra, Indonesia

Abstract

Carbonate buildups in the Arun limestone, North Sumatra, Indonesia, accumulated along the middle shelf margin of the North Sumatra basin. Three major rock types formed the Arun carbonate buildup: (1) coral and coral encrusting-red algae boundstones, (2) bioclastic and large benthic foram packstones, and (3) bioclastic and large benthic foram wackstones. Minor dolomitized rocks are present. Study of 438 feet of core from three wells, the A02, A42, and the A54, reveals that the average porosity is 18 % and the average permeability is 27 md. Good to very good reservoir quality (k>10 md, []>20 %) is associated with grainmoldic (GM), touching vug (VT), and intercrystalline (IX) pore types in the packstones and boundstones. Intermediate to poor quality (0.1<k<10 md, 6<[]<20 %) is associated with separate vug (VS) and GM in the packstones or wackstones, and IX and VS in the boundstones. Dolomitized rocks with no visible pores have poor to very poor quality (k<l md, []<13 %). The quality increases toward the top and the center of the buildup. Better reservoir quality is marked by lower bulk density value and lower gamma-ray value. Dolomitized rocks invariably have higher bulk density (2.7 gr/cm3) and higher gamma-ray values (37 to 40 API) than non-dolomitized rocks (2.27 to 2.49 gr/cm3 and 14 to 35 API). Three geological factors: sea-level change, paleoclimate, and tectonic setting during Lower to Middle Miocene affected the reservoir quality of the Arun buildup. Uplift of the North Sumatra basin during Lower Oligocene provided the foundation for carbonate buildup. Sea-level rise allowed thick accumulation of carbonate sediments, and the following sea-level fall stranded and exposed the buildup to pore-forming diagenesis. Tropical-subtropical setting of the Miocene paleoclimate prevented the development of pore plugging cementation by calcite or anhydrite, and allowed dissolution in the meteoric-phreatic diagenetic environment. As a result of these combined factors, predominantly diagenetic pore types (VT, VS, GM and DC) were formed, transforming the Arun carbonate buildup into an economically important hydrocarbon reservoir.