Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

The Smackover trend of the United States Gulf Coast is a prolific producer of oil and gas. The distribution of porosity is partly controlled by precipitation of late, void-filling calcite. The timing of the formation of these cements is interpreted from fluid-inclusion geothermometry. In general, the fluid-inclusion-filling temperatures closely approximate those of the formation of the calcite and indicate that cementation occurred near the maximum depth of burial. The trapped fluid is a dense calcium-rich brine, consistent with modern formation fluids. The geochemistry of the fluid inclusions reflects the environment of diagenesis; for example, H₂S-rich inclusions occur in cements from deep, dolomitic reservoirs.

The Fateh field, offshore Dubai, has estimated petroleum reserves of 2.3 billion bbl of oil. Production is principally from the Mishrif Formation, an Upper Cretaceous rudist reef complex. Late, coarse-grained, calcite cements partly occlude porosity. These cements contain both aqueous and oil-bearing fluid inclusions. Fluid-inclusion geothermometry indicates temperatures consistent with the geothermal gradient indicating a Miocene age for cement deposition. Since oil was trapped during growth of the calcite cement, petroleum generation and migration are also Miocene in age. Preliminary analyses of the oil trapped in inclusions show it to be different than the oil in the reservoir. The oil trapped in the inclusions may be an early generated, less mature oil.