Organic geochemical indicators of dynamic fluid flow processes in petroleum basins

• Published in: Organic geochemistry 1994-12, Vol.22 (3), p.587-615
• Main Authors: Whelan, Jean K., Kennicutt, Mahlon C., Brooks, James M., Schumacher, Dietmar, Eglington, Lorraine B.
• Format: Article
• Language: English
• Subjects: Applied sciences; Crude oil, natural gas and petroleum products; Earth sciences; Earth, ocean, space; Energy; Eugene Island; Exact sciences and technology; fluid flow; Fuels; gas; Geology and geochemistry. Geological and geochemical prospecting. Petroliferous series; Gulf Coast; Hydrocarbons; Louisiana; migration; migration-fractionation; oil; petroleum; Prospecting and exploration; Prospecting and production of crude oil, natural gas, oil shales and tar sands; Sedimentary rocks
• ISSN: 0146-6380; 1873-5290
• DOI: 10.1016/0146-6380(94)90127-9

A variety of preliminary geophysical, geochemical, and geological observations (Anderson et al., Geophysics, 12–17, April 1991a, b; Anderson, Oil & Gas J. 85–91, 26 April 1993), along with well production data (Schumacher, AAPG Ann. Conv. Abstr., April 1993), have led to the hypothesis that fluid injection into Eugene Island Block 330 (EI-330) in the U.S. Louisiana Gulf Coast may be a continuing episodic process, with the most recent injections having occurred on short geologic time scales (10,000 yr or less). Organic geochemical evidence presented here suggests these processes may occur on very short time scales (years). A process, called “dynamic fluid injection”, was proposed to explain these observations and is envisioned as episodic pressure build-up followed by rapid escape of gas and oil through geopressure and injection into overlying reservoirs. In this contribution, gas and oil compositional data and sidewall core vitrinite reflectance data from EI-330 reservoirs were examined to determine whether they are more consistent with conventional oil and gas alteration processes versus recent dynamic fluid injection. The following EI hydrocarbon patterns are consistent with subsurface oil and gas migration-fractionation having altered the EI-330 oils at sometime in the geologic past: 1. (i) Ratios of n-heptane/methylcyclohexane ( F) plotted against those of toluene/ n-heptane ( B) for EI-330 oils lie in a region of either very high maturity or within a “migrated C7” field typical of an evaporative fractionation event. High maturity was ruled out as the cause of the high F values because ethane vs propane δ 13C values were not consistent with high maturity. Thus, the high F values are proposed to represent the “migrated C7 hydrocarbon” end of an evaporative fractionation event. In comparison, nearby South Marsh Island Block 128 (SMI-128) oils, located on the opposite side of a salt ridge, show higher gas maturities along with a tight clustering of F vs B values consistent with little or no evaporative fractionation. 2. (ii) Vitrinite reflectances are higher in sidewall cores from a fault system thought to feed these reservoirs than in samples away from the fault. 3. (iii) Biomarkers for EI-330 oils are consistent with a source from the same or very similar marine Lower Cretaceous or Jurassic source rocks of almost identical type and maturity. However, significant fractionations in percent sulfur, oil density (API gravity), and percentage of different siz