SOURCE, GENERATION, MIGRATION AND CRITICAL CONTROLS ON OIL VERSUS GAS IN THE DEEPWATER KUTEI PETROLEUM SYSTEM

摘要

The deepwater portion of the Kutei Basin has been an exploration focus for Unocal Indonesia since 1996, which has led to several gas and oil discoveries. Numerous oil, gas and rock samples have been collected during exploration drilling, and analyzed for geochemistry to refine working model(s) of the operating deepwater petroleum system. The geochemical analyses indicate that allochthonous land-plant organic matter is the source of hydrocarbons in the deepwater Kutei Basin. The organic matter in turbidites is dominated by plant leaf fragments (occurring as thin coaly laminae), woody debris and less frequently resin bodies and recycled coaly particles. TOC contents can range from 1 to over 50% with hydrogen indices mostly between 100 and 400 (mg HC/g TOC). The overall kerogen assemblages are type Ⅲ and subordinate type Ⅱ, consistent with a gas condensate to a gas volatile oil petroleum system. No marine algal remains are evident in the deepwater sources, nor are any suggested by oil analyses. Oil/condensate chemistries vary widely but the fundamental genetic makeup of these deepwater liquids shares similar characteristics including (1) high pristane/phytane, oleanane/hopane and bicadinane/hopane ratios, (2) a C_(29)-sterane dominance and the general lack of C_(30)-steranes, (3) high lupanoids, (4) low sulfur and asphaltene, and (5) variable wax content. Gases are mainly thermogenic and the mixing of "biogenic" methane and CO_2 are observed in some shallow Pliocene reservoirs. The generation of oil and gas mostly occurred at "oil window" maturities. Migration fractionation and gas leakage, not source facies, exercised the dominant control on oil versus gas distribution in the Kutei deepwater. It is believed that single-phase hydrocarbon fluids were generated from the Middle- and Lower-Miocene strata, and migrated vertically through faults and fractured shales. Lower pressure-temperature in the upper Miocene and Pliocene reservoirs allowed phase segregation into gas and oil zones. Interbedded oil and gas resulted from multiple episodes of migration charging. Iterations of migration, fractionation and gas leakage enriched oils in an otherwise gas-rich basin.