Pematang-Sihapas Petroleum System of Central Sumatra

B. J. Katz and WM. C. Dawson

The Pematang-Sihapas (!) petroleum system of Central Sumatra is one of the most important lacustrine oil systems in Southeast Asia. The Brown Shale Formation (lacustrine) of the Pematang Group has sourced (approx.) ~60 × 10⁹ BO. Reservoirs are developed within the uppermost Pematang Gp. (Paleogene) and superjacent Sihapas Group (Early Miocene). Sihapas sandstones are the principal reservoirs of this system. Giant fields (e.g., Minas and Duri) having Sihapas (marine sandstone) reservoirs occur principally along the eastern margins of the rift basins. Smaller fields having Pematang (nonmarine sandstone) reservoirs are confined to the rift basins.

The Pematang Group was deposited in a series of small grabens. It exhibits a tripartite arrangement: basal (Lower Red Bed) fluvial/alluvial unit; medial (Brown Shale) lacustrine unit; and upper (Upper Red Bed) fluvial/alluvial unit.

Source rock attributes of the Brown Shale are highly variable. Pyrolysis yields of samples containing >=1.0 wt.% TOC range up to (approx.) ~120 mg HC/g rock, with a mean of (approx.) ~25.3 mg HC/g rock. Differences in kerogen reflect variations in preservation rather than organic input.

The overlying Menggala Formation (basal transgressive unit of Sihapas Group) has an average porosity of >20% and an average permeability of 1500 md. The amount of sandstone (gross) in the Menggala Formation typically exceeds 50%. Menggala strata consist of well-sorted quartzose to subarkosic sandstones. Menggala sandstones are the most prolific hydrocarbon reservoirs in Central Sumatra.

Many of the oil fields in Central Sumatra are associated with structures formed during a mid-Miocene compressional event.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California