ABSTRACT: Paleofluid Flow Paths--Evidence from Thermal Maturity Mapping, Greater Green River Basin, Wyoming and Utah

LAW, BEN E., U.S. Geological Survey, Denver, CO

Paleofluid flow is an important but often elusive component of hyrocarbon exploration. An underutilized method of gaining insight into this problem is thermal maturity mapping. For example, thermal maturity mapping in the Greater Green River Basin of Wyoming and Utah has revealed areas of unusually high and low thermal maturity that most likely resulted from thermal perturbations associated with ascending and descending fluids. The area of high thermal maturity is coincident with the Patrick Draw oil field (Upper Cretaceous Almond Formation field), located east of the Rock Springs uplift on the Wamsutter arch. The area of low thermal maturity is coincident with the Bridger Lake and Lucky Ditch oil fields (Lower Cretaceous Dakota Sandstone fields), located near the intersection of the oxa arch and the Uinta thrust fault.

The area of high thermal maturity is centered on the Patrick Draw field. Mean random vitrinite reflectance (Rm) values in the Almond Formation, at a depth of about 5000 ft, range from 0.48% immediately adjacent to the high area, to 0.68% within the area. I suggest that faults and fractures associated with a northeast-trending zone of known faults have provided pathways for relatively hot, vertically flowing oil-bearing fluids. The passage of these hot fluids through these faulted and fractured rocks is indicated by high levels of thermal maturity. In contrast, the south end of the Moxa arch is characterized by low thermal maturity and a low thermal gradient. At depths of about 15,000 ft, the vitrinite reflectance of the Dakota Sandstone is only 0.60%, and the present-day thermal gradi nt is about 1.0 degrees F/100 ft. Vitrinite reflectance well profiles in this area are nearly vertical (low gradient). These characteristics, in conjunction with compositional variations of formation water and oil gravity, indicate that these unusually low thermal conditions are the result of temperature suppression caused by descent of cool meteoric water into the basin through faults and fractures along the structurally deformed north flank of the Uinta Mountains. Interpretation of vitrinite reflectance data for these two areas has provided important clues to the origin of the oil accumulations that otherwise might not be evident.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)