ABSTRACT: Paleosols as top seals for nonmarine petroleum systems, Central Sumatra Basin, Indonesia

Almon, William R., and William C. Dawson , Texaco Upstream Technology, Houston, TX

Paleosols interstratified with nonmarine (fluvial and lacustrine) and marginal marine (estuarine) lithofacies successions in the Central Sumatra Basin were analyzed using high-pressure mercury injection capillary pressure (MICP) to determine sealing capacity. Petrographic, SEM, and XRD analyses document the texture and mineralogy of the paleosols. A thick regionally extensive paleosol marks the 25.5 ma sequence boundary throughout the Central Sumatra Basin. All samples exhibit well-developed mega- and microscopic pedogenic features.

The analyzed paleosol samples range from fine-grained argillaceous sandstones to silty claystones. Illite is the dominant (mean 62%) clay mineral. Detrital quartz is the major (mean 61%) silt-sized component. Kaolinite and authigenic siderite occur in accessory amounts. The clay matrix is densely compacted, cemented with quartz or carbonates, and partially recrystallized. Porosity ranges from 1.5% to 9.7%, and permeability varies from 0.2 md to 0.007 md. MICP analyses suggest that these paleosols are good to excellent seals capable of retaining columns ranging from about 500 to 4,600 feet (oil) and from 600 feet to 5900 feet (gas). Column heights vary with changes in API gravity, temperature, and fluid density.

Sealing capacity correlates with total clay content and position within the soil zone. The C-horizons are excellent seals, and B-horizons offer somewhat less sealing capacity. Hydrocarbons can leak across paleosol horizons along faults of fractures or where breached by fluvial or tidal (incised valley) channels. A thick mature paleosol along the 25.5 ma sequence boundary appears to have focused migration toward the eastern margin of the Central Sumatra Basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia