Janell D. Edman

Influence of overthrusting on maturation of hydrocarbons in Phosphoria Formation, Wyoming-Idaho-Utah overthrust belt

Geochemical analyses were performed on 11 Phosphoria Formation samples collected throughout the Wyoming-Idaho-Utah Overthrust belt. Maturation models incorporating the thermal effects of thrust faulting with Lopatins techniques were then used to interpret these geochemical data. In particular, four of the sample sites were selected for detailed thermal modeling. These four sites include one sample site in an area unaffected by thrusting, two sites located in the hanging walls of major thrusts, and one site in the footwall of the Darby thrust. The detailed thermal modeling shows overthrusting did affect the thermal history of the Phosphoria Formation. The effect of thrusting was to cool the overriding sheet and warm the sediments being overridden. Thus, the hanging wall samples had a different post-thrusting thermal history than the footwall sample. Cooling of the hanging wall slowed post-thrusting maturation in the two thrust-sheet samples. In contrast, the footwall sample was exposed to higher temperatures after thrusting, and the maturation process was accelerated. Results of this study indicate that tectonism influences the thermal history of the rocks and should be included in any reliable source rock evaluation program involving overthrust strata. A simple evaluation based only on the thickness of the overburden, in general, will not be sufficient to explain fully the maturation data. In the absence of actual geochemical information such as Ro, TAI or CAI, the models described in this paper appear to be sufficiently accurate to give a reasonable estimate of thermal maturity of a potential source bed.

Thrust Faulting and Hydrocarbon Generation: REPLY

This paper is a rebuttal of an earlier paper trying to refute the claims of these authors. They believe this reply illustrates that the simplifications used by Warner and Royse (1987) in their model are generally inappropriate to describe the thermal history of units in the Western Overthrust belt, as well as other overthrust regions. Although their shortcuts produce results that are generally consistent with measured maturity values for some specific modeling sites, the differences in maturation history and the mismatch for the general thrust case make their model a less effective predictive tool. At this stage in the understanding of overthrust thermal processes, you cannot predict a priori when the thermal effects of thrusting will be significant and when they will be minimal. Defining the exact relationship between thrusting and hydrocarbon accumulations in overthrust areas requires additional work and data collection. In particular, workers must consider case histories where the hanging wall contains thick sequences of resistant strata. By incorporating additional constraints, instead of returning to the simplified assumption of constant thermal gradients in tectonic regions, the understanding of the evolution and maturation history of thrust belts is improved.

Mechanism for Enhanced Reservoir Porosity Generation in Tripolitic Chert: ABSTRACT

Tripolitic chert is present in many Rocky Mountain basins. In the Green River basin, enhanced porosity in tripolitic chert comprises a significant portion of the porosity in the Ericson Sandstone. Prediction of enhanced reservoir porosity within tripolitic chert is dependent on understanding the mechanism for creation of this porosity. Dissolution experiments were performed on selected chert samples to determine the causal mechanism. Chert samples used in the dissolution experiments were from the Phosphoria Formation in the Overthrust belt. Paleocurrent directions and the presence of sponge spicules and apatite in the Ericson cherts all indicate a Phosphoria source within the thrust belt for much of the Ericson chert. Both acid-treated and untreated chert fragments were placed in organic acid solutions. Results of the experiments show it is the 25-30% carbonate fraction within the chert that dissolves to create tripolitic chert. Silica solubility was not affected by the organic acids. The opportunity to dissolve calcite and dolomite within detrital chert fragments exists at several times during progressive burial. Depending on the fluid chemistry, carbonate could be dissolved by an early pulse of organic acids generated prior to hydrocarbon migration. This would be End_Page 847------------------------------ the ideal situation in which enhanced porosity is created immediately before migration. Later in the burial history, after thermal decarboxylation of organic fluids produces CO2, a second opportunity exists to create enhanced porosity. Using these diagenetic concepts, it may be possible to predict enhanced reservoir porosity within tripolitic chert. End_of_Article - Last_Page 848------------