Brian J. Cardott

Detection of natural weathering of Upper McAlester coal and Woodford Shale, Oklahoma, U.S.A.

Abstract Natural weathering refers to the natural oxidation of organic and inorganic matter in rocks at the surface and in the shallow subsurface. The objective of this study is to apply routine geochemical analyses (including organic petrography) to coal and shale samples, extending from the weathered to unaltered zones, to illustrate the effects of weathering on organic matter in coal and shale. Some significant observations from the Upper McAlester coal and the Woodford Shale are: (1) a sulfate index (SI), using sulfur-form analysis data that reveals the transformation of pyrite to sulfate minerals, is a very sensitive weathering indicator; it increased in weathered samples; (2) Rock-Eval pyrolysis data, such as hydrogen index (HI) and oxygen index (OI), are very sensitive to weathering; OI/HI increased in weathered samples; (3) vitrinite reflectance decreased in weathered samples; and (4) petrographic signs of weathering include microfractures, pitted surface, dark reaction rims, and high relief. Sulfate index and petrographic signs of weathering should be used on surface and shallow subsurface rock samples as an indication of the extent of weathering; the validity of other analyses can then be evaluated accordingly.

Thermal Maturation by Vitrinite Reflectance of Woodford Shale, Anadarko Basin, Oklahoma

The Woodford Shale (Upper Devonian-Lower Mississippian), a black shale widely regarded as an important hydrocarbon source rock, has attained thermal maturity to postmaturity with respect to the generation of liquid hydrocarbons in most of the Anadarko basin in western Oklahoma. Untreated whole-rock well cuttings and core material from selected 10-ft intervals of the Woodford Shale in 28 wells were examined under reflected white light. Thermal maturation was determined by measuring the reflectance in oil immersion (Ro) of first-generation vitrinite particles. A minimum of 45 reflectance measurements were recorded for each well representing one or more sample intervals. The Woodford Shale was sampled at depths from 5,060 ft (1,542 m) in the northeastern shelf to 25,115 ft (7,655 in the deepest part of the basin. A systematic increase in mean vitrinite reflectance (meanRo) with depth was observed over much of the Anadarko basin. From northeast to southwest across the basin, mean Ro increases from 0.48 to 2.61%. High temperatures have raised the Woodford Shale to anthracite rank in two wells (4.29 and 4.89% Ro) in the deep Anadarko basin. An isoreflectance map for the Woodford Shale indicates those areas of the basin where the thermal history of the shale is optimum for preserving liquid hydrocarbons (mean Ro of 0.5 to 2.0%). Computer-generated plots of vitrinite reflectance versus depth provide a regression equation that predicts the vitrinite reflectance of the Woodford Shale at any depth in the Anadarko basin and determines the amount of erosion.

Source rock/dispersed organic matter characterization—TSOP research subcommittee results

Abstract Because sedimentary organic matter consists of a diverse mixture of organic components with different properties, a combination of chemcial and petrographic results offers the most complete assessment of source rock properties. The primary purpose of this Society for Organic Petrology (TSOP) subcommittee is to contribute to the standardization of kerogen characterization methods. Specific objectives include: (1) evaluation of the applications of different organic matter (petrographic) classifications and terminology, and (2) integration of petrographic and geochemical results. These objectives were met by completing questionnaires, and petrographic, geochemical and photomicrograph round-robin exercises. Samples that were selected for this study represent different petrographic and geochemical properties, and geologic settings to help identify issues related to the utilization of different classifications and techniques. Petrographic analysis of the organic matter was completed using both a prescribed classification and the individual classification normally used by each participant. Total organic carbon (TOC), Rock-Eval pyrolysis and elemental analysis were also completed for each sample. Significant differences exist in the petrographic results from both the prescribed and individual classifications. Although there is general agreement about the oil- vs gas-prone nature of the samples, comparison of results from individual classifications is difficult due to the variety of nomenclature and methods used to describe an organic matter assemblage. Results from the photomicrograph exercise document that different terminology is being used to describe the same component. Although variation in TOC and Rock-Eval data exists, geochemical results define kerogen type and generative potential. Recommendations from this study include: 1. (1) A uniform organic matter classification must be employed, which eliminates complex terminology and is capable of direct correlation with geochemical parameters. 2. (2) A standardized definition and nomenclature must be used for the unstructured (amorphous) organic matter category. Subdivisions of this generalized amorphous category are needed to define its chemical and environmental properties. 3. (3) Standardized techniques including multimode illumination, types of sample preparations and data reporting will help eliminate variability in the type and amount of organic components reported.

Organic Petrology of Epi-Impsonite at Page, Oklahoma, U.S.A.

Abstract Impsonite (asphaltic pyrobitumen) occurs as fracture-filling veins cutting massive sandstone in the frontal Ouachita Mountains near Page, Oklahoma. The Page impsonite formed from low-temperature alteration of crude oil. Mean maximum bitumen reflectance in oil immersion ( R max ) of seven samples is 1.41–1.96%. Mean apparent bireflectance of these samples is 0.15–0.54%. The Page deposit classifies at the upper end of epi-impsonite in the generic classification for solid bitumen, based on physical, chemical, and optical characteristics, and as post-oil with unlimited migration in the genetic classification for solid bitumen.

Nature of Migrabitumen and Their Relation to Regional Thermal Maturity, Ouachita Mountains, Oklahoma

Abstract Two grahamite and three impsonite localities are within an 82-km-long segment of the Ouachita Mountains of southeastern Oklahoma. Grab samples were collected to study the petrographic and geochemical characteristics of the migrabitumen at the grahamite-impsonite transition and the relation of the migrabitumen to the regional thermal maturity pattern. Maximum and random bitumen reflectance values increased from 0·75 to 1·80% from west to east, consistent with the regional thermal maturation trend. Mean bireflectance values increased from 0·04 to 0·38%. The two grahamite samples are classified at the grahamite-impsonite boundary with conflicting petrographic (bitumen reflectance) and bulk chemical (volatile matter) maturity indicators. The regional maturation trend, based on vitrinite reflectance and bitumen reflectance values, was confirmed by a detailed geochemical investigation of bitumen extracts. Although biomarker analyses were influenced by extensive biodegradation effects, molecular paramet...

Thermal Maturation by Vitrinite Reflectance of Woodford Shale, Arbuckle Mountains, Oklahoma: ABSTRACT

Vitrinite reflectance was measured on 40 grab samples from outcrops of the Woodford Shale (Upper Devonian-Lower Mississippian) collected near the Washita Valley fault in the Arbuckle Mountains in south-central Oklahoma. Samples are widely distributed along 40 km. Sample localities range from 60 m to 7.63 km from the Washita Valley fault. Well-indurated shale samples were collected from below the outcrop surface to reduce the effect of weathering on vitrinite reflectance. Vitrinite reflectance values were measured from standard kerogen concentrate pellets. Implications of the data specific to the Arbuckle Mountains include the Woodford Shale is immature to marginally mature with respect to the generation of liquid hydrocarbons; high heat flow associated with the rifting stage of the southern Oklahoma aulacogen was diminished by Late Devonian; the Woodford Shale was never deeply buried; and frictional heating from the Washita Valley fault did not affect the temperature field significantly.