Abstract

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 generated ~60 × 10⁹ barrels of oil. Sihapas (Early Miocene) 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 sub-basins. Smaller fields having Pematang (nonmarine sandstone) reservoirs are confined to the deeper troughs.

The Pematang Group was deposited in a series of small grabens. It exhibits a tripartite stratal architecture: basal (Lower Red Bed) fluvial/alluvial unit; medial (Brown Shale) lacustrine unit; and upper (Upper Red Bed) fluvial/alluvial unit. The Upper Red Bed unit is the main Pematang reservoir. Source rock attributes of the Brown Shale are highly variable. Pyrolysis yields of samples containing > 1.0 wt.% TOC range up to ~120 mg HC/g rock, with a mean of ~25.3 mg HC/g rock. The oil-proneness of the kerogen also varies. The more oil-prone portions of the unit appear in the upper portion of the stratigraphic unit and in the fully lacustrine facies (i.e., poor source rock development occurred in lake margin, deltaic and fan delta facies). Geochemical differences also exist among the different sub-basins.

The Pematang Group is disconformably overlain by the Menggala Formation (basal transgressive unit of Sihapas Group). Menggala strata consist of well-sorted quartzose to subarkosic sandstones having an average porosity of > 20% and an average permeability of 1500 md. Menggala sandstones are the most prolific hydrocarbon reservoirs in Central Sumatra.

Many of the oil fields in Central Sumatra are associated with paleohighs, drag folds, and post mid-Miocene inversion. Hydrocarbon generation was initiated during the Miocene and continues currently in some parts of the basin.