ABSTRACT The lower Permian Wolfcamp Shale in the Permian Basin is a major unconventional resource play composed of organic-rich, siliceous and calcareous mudstones interbedded with carbonate turbidites and debrites. Using two cores that comprise the Wolfcamp Shale near the eastern margin of the Midland Basin, this study reconstructs the complex diagenetic history of both the mudstone and carbonate facies. These cores were analyzed using petrographic and SEM techniques to test if the Wolfcamp Shale was an open or closed system and to characterize diagenetic processes that impact reservoir characteristics, such as porosity types, porosity distribution, permeability pathways, and mechanical brittleness. Early, middle, and late phases of chemical diagenesis are defined in this study. Mineral precipitation and dissolution events occur from the passage of fluids through both interstitial and fracture pore space. Early authigenic mineral precipitation (calcareous and phosphate concretions, sphalerite, barite, framboidal pyrite, quartz, dolomite, and ferroan dolomite) resulted in destruction of primary porosity within the mudstone facies, before and during the mechanical compaction event. Destruction of porosity in the carbonate turbidites facies occurred through carbonate cementation (calcite, ferroan calcite, dolomite, and ferroan dolomite) during early to middle diagenesis. An episode of dissolution and dolomitization in the carbonate facies resulted in the creation of moldic and intercrystalline porosity respectively. Within mudstones intercrystalline porosity is observed between pyrite framboids and clay sheets of chlorite. Diverse fracture types occur in all facies within the Wolfcamp Shale and play a critical role in the migration of diagenetic fluids and hydrocarbons. Horizontal fractures are filled by “beef”-type calcite, and vertical fractures are filled with equant calcite and/or celestine-barite. Mineralized fractures contain porosity, some of which contain ferroan dolomite rhombs within pores, which supports diagenetic fluid movement through fractures after an initial stage of mineralization. Fluid inclusion data suggest that some mineralized fractures acted as fluid conduits for externally derived, warm, high-salinity brines, suggesting the Wolfcamp Shale was an open system during it burial history.

ABSTRACT The Mississippian limestone is a petroleum exploration target in northern Oklahoma, and diagenetic events are significant factors in controlling porosity. In this study, paleomagnetic data, supported by petrographic results, were used to determine the origin and timing of diagenetic events in five unoriented cores from northern Oklahoma. Petrographic analysis indicates a complex paragenetic sequence, which includes precipitation of sphalerite and baroque dolomite. Thermal demagnetization removes a low-temperature viscous remanent magnetization (VRM) and a chemical remanent magnetization (CRM) in magnetite. An attempt was made to orient the cores using the VRM but this resulted in a streaked distribution of declinations. The inclinations of the CRM in the specimens in the five cores are similar (mean = −2.6°) and the age of the CRM was determined by comparing the inclinations with the expected inclinations for the study area. This indicates remanence acquisition in the Permian (~310–290 Ma). This is consistent with dates for mineralization in the nearby Tri-State MVT deposit and for a hypothesized Permian hydrothermal alteration event in the study area. The age of the CRM and the presence of sphalerite and baroque dolomite suggest that the CRM was acquired via hydrothermal fluids in the Permian.

“…the frustration of discovering an unusually good exposure or feature only to have it quarried or covered in succeeding weeks is disheartening. On the other hand, quarry advance enables one to project the geology from time to time, which helps one to fill in the three-dimensional puzzle.” — Gutschick (1972) ABSTRACT We summarize and then build on the three decades of geological mapping and analyses done by Ray Gutschick at the Newton County (Kentland) quarry. We present our own new data and ideas on the kinematics and significance of radial faults, shock metamorphism, petrography and diagenesis of impact breccia dikes, impactite geochemistry, and a preliminary new paleomagnetically determined Jurassic age for the crater. We list and describe the stops for this field excursion.

Abstract The Devonian Alamo Breccia is a thick (<30–130 m) unit, interpreted as a bolide impact deposit, which is bracketed by marine carbonates. Samples were collected within the breccia and above/below the breccia for a contact test to determine if the breccia acted as a conduit for fluids that could have caused the widespread chemical remanent magnetizations (CRMs) present in Palaeozoic Era rocks in Nevada. The carbonates above, below and in the breccia contain a Cretaceous Period syn-tilting CRM that resides in pyrrhotite and a pre-tilting late Palaeozoic Era CRM that resides in magnetite. The contact test is negative. Despite these results, diagenetic alteration by externally derived fluids is interpreted as the most likely mechanism of remagnetization. This hypothesis is supported by 87 Sr/ 86 Sr values in the breccia and surrounding rocks that suggest alteration by fluids with a radiogenic signature. The fluids were not localized in the breccia but are interpreted to have moved pervasively through the rocks. The results differ from some other studies that found that fluids caused localized CRMs around fluid conduits.

Abstract Paleomagnetic, petrographic, and geochemical results, as well as field relationships, are used to relate Late Paleozoic chemical remanent magnetizations (CRMs) to the migration of basinal fluids in Ordovician carbonates in the Arbuckle Mountains, southern Oklahoma. The Viola Limestone contains a pervasive Pennsylvanian synfolding CRM residing in magnetite and a localized Permian CRM which resides in hematite and occurs in alteration zones around veins mineralized with calcite and Mississippi-Valley-type oxides and sulfides. The vein mineralization precipitated from basinal fluids that were warm, saline, and radiogenic. Radiogenic 87 Sr/ 86 Sr ratios of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggest that the basinal fluids were also responsible for alteration in the limestones. The coincidence of the geochemical and remagnetization trends suggest that the Permian CRM dates the migration of the basinal fluids in the veins. Geochemical results from the Viola with pervasive CRM indicate that it is relatively unaltered, with no evidence for radiogenic basinal fluids. This suggests that a mechanism that does not necessarily require exotic externally-derived fluids is needed to explain the origin of the pervasive CRM. Liesegang-banded carbonate around calcite-filled fractures in the Kindblade Formation also contains a Permian CRM residing in hematite. The fluid that precipitated the hematite liesegang bands emanated from the fractures and, based on geochemical results, was basinal in origin. The results of this study suggest that basinal fluids migrated through the carbonates in the Arbuckle Mountains during the Permian, although perhaps in several episodes. The flow of basinal fluids was focused in veins and only locally altered the host carbonates.

Abstract Paleomagnetic, rock magnetic, petrographic, and geochemical studies indicate that hydrocarbons can cause either an increase or decrease in the magnetization in sedimentary rocks. For example, hydrocarbon-impregnated Permian calcite speleothems in southwestern Oklahoma contain a Permian chemical remanent magnetization (CRM) that resides in magnetite. A positive relationship between extractable organic matter and the natural remanent magnetization (NRM) suggests that the chemical conditions created by the hydrocarbons caused precipitation of the magnetite and acquisition of the associated CRM. There is no correlation, however, between percent asphaltenes and NRM in the speleothems. In addition, bitumen speleothems with high NRMs are, in general, less extensively biode-graded. These results suggest that a chemical process, and not biodegradation, is the mechanism for magnetite authigenesis in speleothems. The results from the speleothems suggest that hydrocarbons can cause acquisition of magnetization that can be dated using paleomagnetic analysis. Development of this dating approach, however, requires more work to better understand the mechanism(s) of magnetite precipitation. Studies of red bed and hydrocarbon-impregnated samples from cores of the Lyons Sandstone in the Denver Basin and from outcrops of the Maroon Formation (Schoolhouse Member) in northwest Colorado indicate that while hydrocarbons can cause precipitation of magnetite, they can also reduce the NRM by dissolution of hematite. This has implications for the various types of magnetic prospecting techniques that have been proposed.