Diagenetic History of the Phosphoria, Tensleep and Madison Formations, Tip Top Field, Wyoming

Abstract Petrographic and geochemical data from cores in the Wyoming thrust belt are used to relate maturation and migration of Phosphoria Formation organic material to the evolution of porosity in the Tensleep and Madison Formations. Observed paragenetic sequences for each formation indicate that all three formations exhibit two distinct phases of diagenesis. These two phases are delineated chronologically by the simple designations of “early” and “late.” The period of early diagenesis was responsible for the pore network present in the Tensleep and Madison during migration of Phosphoria-sourced hydrocarbons. This early diagenetic phase exhibits a strong dependence on depositional environment, and is distinct for each formation. In contrast to this early diagenesis, the late diagenesis is consistent and similar for all three formations. This is interpreted to be the result of a relatively uniform sequence of fluids migrating through a pervasive Sevier fracture system that penetrated the entire 2000-ft thick stratigraphic interval of interest. The late diagenesis determined the present porosity configuration in these formations. The sequence of organic reactions related to the maturation and migration of Phosphoria organic material also influenced late diagenesis in all three formations. Generation of hydrocarbons was the first event in the late diagenetic period of each formation. Abundant organic material in the Phosphoria Formation entered the liquid window just prior to the emplacement of the Darby thrust during the Paleocene. Fracturing associated with thrusting during the Sevier orogeny provided conduits for the migration of Phosphoria hydrocarbons. Subsequent deep burial of the source and reservoir rocks beneath the Darby plate resulted in thermal degradation of the hydrocarbons, and liquid hydrocarbons were altered to solidified bitumen, methane, CO 2, and H 2S. Both the solidified bitumen as well as late dolomite cement severely damaged the early pore systems in the Tensleep and Madison. However, additional by-products of the thermal degradation process (CO 2and H 2S) dissolved remnant fossil fragments in the dolomite facies of the Madison to produce late, moldic porosity. The end result of this complex sequence of events is that at present the Phosphoria and Tensleep are tight and extremely well indurated, while the Madison possesses late moldic porosity from which to produce the methane reserves in the Tip Top field.