Abstract

There are fundamental differences in the physical/chemical properties of the fluids and the rocks between the anomalously pressured, compartmentalized Cretaceous Muddy Sandstone - Mowry Shale and the normally pressured, compartmentalized Pennsylvanian-Permian Minnelusa Formation in the Powder River Basin (PRB) of Wyoming. These differences account for the Muddy-Mowry being stratigraphically compartmentalized and abnormally pressured, and the Minnelusa being similarly stratigraphically compartmentalized but not generally abnormally pressured. This study compares geochemical data from the Muddy Sandstone - Mowry Shale and the Minnelusa Formation in the PRB, in an attempt to explain how a compartmentalized, normally pressured sandstone can exist stratigraphically below a compartmentalized, overpressured sandstone.

The Mowry Shale associated with the Muddy Sandstone is a regionally significant source of oil and gas (up to 5 wt% TOC), whereas the locally discontinuous and thin shales associated with the upper and lower Minnelusa are regionally insignificant sources. Portions of the middle Minnelusa, which is stratigraphically more compartmentalized, are also a richer petroleum source, and may behave like the Muddy-Mowry system. The Muddy-Mowry system shows a significant acceleration of thermal maturity parameters (nuclear magnetic resonance, vitrinite reflectance, production index, and clay diagenesis) over the depth interval that contains abnormally pressured compartments; the Minnelusa shows no such trend. The Muddy Sandstone shows a marked increase in formation water salinity and alkalinity with depth at the top of the abnormally pressured compartments; the Minnelusa Formation shows no discernable trend with depth in formation water chemistry. Relative to the Minnelusa, the Muddy-Mowry was buried rapidly, allowing relatively quick, in-situ generation of oil and gas. The Minnelusa, which for the most part has no closely associated petroleum source, was buried near the surface in the meteoric water regime for ~120 Ma. It developed initially as a hydrologic flow regime and has not apparently changed. In contrast, as the Muddy-Mowry system evolved, oil and gas were generated and migrated into adjacent sands where they expelled formation waters until low-permeability rocks were encountered and capillary seals were activated, establishing pressure compartments. The Minnelusa is generally too poor in organics to be a source of significant liquid or gaseous petroleum; therefore, the oil-in-place migrated into the Minnelusa from a distant source; it has been suggested that a long (>100km) migration distance is involved in charging much of the Minnelusa (cf. Stone, 1967). Although rich shales have been identified locally in the middle Minnelusa (Clayton and Ryder, 1984), they are discontinuous, and are volumetrically insufficient to have generated all the oil in the Minnelusa. Also, the oil in the Minnelusa has had long reservoir residence time at moderate temperature; both conditions are known to cause a very slow rate of cleavage of short-chain aliphatic side chains from petroleum - these cleaved side chains are a major source of early thermogenic gas. The slow rate of generation allowed dissipation of the gas, whereas a pulse of this early gas from the oil → gas reaction is known to have activated capillary seals in the Muddy-Mowry. Where explored, the Minnelusa is not generally buried deeply enough for long-chain hydrocarbons to have cracked to gas (gas generation may be necessary for three-dimensional closure of capillary seals). These facts and comparisons may explain why pressure compartmentalization in Minnelusa reservoirs does not occur to any great extent in the portions explored for oil and gas.

We hypothesize that Minnelusa gas plays that are (1) where the middle Minnelusa is compartmentalized and a rich source rock or (2) deep and hot enough to thermally crack long-chain hydrocarbons to gas and characterized by three-dimensional closure of capillary seals will contain abnormally pressured compartments.