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ABSTRACT: Correlative Nonmarine (Paleosol) and Marine (Hardground) Top Seals Developed at Sequence Boundaries

William C. Dawson and William R. Almon , Texaco Upstream Technology, 3901 Briarpark, Houston TX 77084, (713) 954-6086 / FAX (713) 954-6113

Sequence boundaries, represented in conventional cores as marine hardgrounds (cemented and bored diastems) that are correlative with up-dip paleosol horizons, can have excellent sealing capacity for hydrocarbons. Hardgrounds act as bottom seals for lowstand or basal transgressive sandstone reservoirs. Concurrently, a hardground may form a top seal for a subjacent marine sandstone package. In the absence of a hardground, shales occurring in the upper third of transgressive systems tracts offer excellent sealing capacity for underlying reservoirs. Paleosols are top seals for fluvial, lacustrine, and estuarine sandstone reservoirs. Regionally extensive paleosol horizons can influence hydrocarbon migration pathways. Hydrocarbons leak through paleosol horizons along faults of fractures or where breached by fluvial or tidal (incised valley) channels.

Hardgrounds are effective seals because of early submarine cementation (calcite or siderite) during periods of reduced sedimentation. High-pressure mercury injection capillary pressure (MICP) analyses indicate 10 percent mercury saturation occurs at 13,000 to 16,000 psia above an analyzed hardground and from 21,000 to about 26,000 psia below, indicating the ability of this hardground to maintain a significant pressure difference. Analyzed paleosol horizons yield 10 percent mercury saturation values ranging from 5,000 to 15,525 psia, indicating good to excellent sealing capacity. The sealing capacity of paleosols increases with increases total clay content and/or increased quartz cementation. Calculated oil columns for the studied hardgrounds range from 1,300 to 7,500 feet and 750 to 4,400 feet for the paleosol horizons. Calculated column heights vary with changes in API gravity, temperature, and fluid density.

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