Print this pageRecord of Source-Generated Overpressures, Venezuela
Peter J. Vrolijk, Robert J. Pottorf, and William B. Maze
Fluid pressures affect migration of oil, gas, and water in continental margins. Burial and thermal history models describe the degree to which undercompaction or thermal expansion of fluids contribute to fluid pressure histories, but it is more difficult to evaluate how source-terms, such as oil yield or mineral dehydration reactions, impact paleo-fluid pressures. In this study, we document how a thick, maturing source rock helped create near-lithostatic fluid pressures that generated overpressures in reservoir rocks. We analyzed abundant oil-filled and rare aqueous fluid inclusions in calcite-filled fractures in the La Luna Fm. source rock and in the underlying Cogollo Gp. carbonate; reservoir in the W. Maracaibo Basin, Venezuela. Homogenization temperatures (Th) of oil-filled inclusions range from 25-42°C in the La Luna Fm. and from 25-105°C in the Cogollo Gp., and associated gravities (determined from fluorescence properties) range from 28-43°API and 17-45°API, respectively. Integration of Th with the burial and thermal history of the sampled horizons leads to the conclusion that fractures in the La Luna Fm. formed under near-lithostatic fluid pressure conditions in the presence of a gas-charged oil. Th values from fractures in the Cogollo Gp. are higher than in the La Luna Fm and become more variable with increasing depth below La Luna. We interpret those fractures to have formed under lower fluid pressure conditions and/or with a less gas-charged oil than for La Luna. This interpretation of the dis ribution of paleo-fluid pressures is supported by the observation of modern inverted fluid pressure gradients between upper and lower Cogollo Gp. reservoirs. Thus late expulsion of a gas-charged oil created near-lithostatic fluid pressures in the La Luna Fm. source rock, and those fluid pressures bled downward through fractures into the adjoining reservoir rocks, contributing to the overpressures we observe today.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California