Reservoir quality, textural evolution, and origin of fault-associated dolomites

Abstract

Fault-associated dolomitized carbonates are proven hydrocarbon reservoirs in the subsurface; yet their origins and spatial variability in reservoir quality are poorly understood. Fault-associated dolomitization has affected a 4–8-km (2.5–5-mi)-wide strip of inner-platform carbonates of the Oligocene–Miocene Taballar Limestone, exposed onshore northeastern Borneo, where they are juxtaposed against deep-marine shales of the Eocene Maliu Mudstone. On textural grounds (planar dolomites postdating compaction) and from the predominance of monophase aqueous inclusions, replacive dolomite rhombs and later dolomite cements appear to have formed at about 50–60C and at shallow burial depths up to 0.5–1 km (0.3–0.6 mi). Isotopic and inclusion data indicate that the most likely dolomitizing fluid was dominated by Neogene seawater (for dolomites, 18OV-PDB = 5.9 to 10.3; [consistent with the precipitation from southeast Asian Oligocene–Miocene seawater {18O = 1.5 to 4.2} at temperatures of 40C up to a maximum of about 50–75C], 87Sr/86Sr = 0.708566–0.708697; from Tm = 2.1–3.4 wt.% NaCl). However, replacement and remobilization of the precursor limestone also contributed to the isotopic signature of the dolomites (87Sr/86Sr = 0.708115–0.708197), whereas an evolved formation water component cannot be ruled out. Dolomitizing fluids used faults and fractures as conduits to move into and alter the limestone, migrating farthest into the most permeable strata. The likely driver for fluid movement was convective flow caused by nearby Neogene igneous activity, perhaps in combination with tectonically induced hydrologic drive related to fault reactivation.Close to the main fault, late-stage dolomite cements occlude pore throats, reducing porosity (5%) and permeability (5 md). The best reservoir quality occurs in medium- to coarsely crystalline idiotopic mosaics of dolomite that have completely replaced the limestone 0.5–2 km (0.3–1.2 mi) away from the main fault where the late-stage cements did not form. These dolomites have 12–20% intercrystalline porosity and moderate to good permeability (tens of millidarcys). Fracturing has had variable impact on reservoir quality. Where late fractures remained open, permeability is enhanced (tens to hundreds of millidarcys), whereas brecciation and fault gouges result in sealing and reduced permeability. The textures, reservoir quality characteristics, and mechanisms of dolomitization show similarities to the few other detailed studies in the literature. This study refines the types of predictive reservoir models used in hydrocarbon exploration for subsurface faulted and dolomitized carbonates and increases the understanding of dolomitizing mechanisms of equatorial carbonates.