SEGALL, MARYLIN P., Earth Sciences and Resources Institute, University of South Carolina, Columbia, SC; and J. K. RAFALSKA, Conoco Inc., Ponca City, OK

ABSTRACT: Effects of Burial Depth and Thermal Maturity on Kerogen-Rock Microfabric

A suite of organic-rich, fine-grained sediments and rocks has been examined by Rock-Eval, vitrinite reflectance and fluorescence, scanning electron microscopic, transmission electron microscopic, and X-ray diffraction techniques to determine the evolution of source rocks. The samples from marine depositional environments span a range of kerogen maturity: immature (%Ro 0.2); early mature (%Ro 0.5); mature (%Ro 0.6-1.3); very mature (%Ro 1.3-2.0); and supermature (%Ro>2.0). Kerogen quality (Type II, HI: 150-400; OI:<25)) and organic matter concentrations (TOC from 0.2% to 15%) characterize the organic facies. All samples display some degree of fine-scale interlaminations, probably inherited from the time of deposition. These micro-interlaminations consist of resistant and less-res stant microlaminae. Resistant microlaminae are characterized by densely-packed grains of uniform size (approx. 2 micrometers) with stepped face-face contacts. Less-resistant microlaminae consist of variable grain size (clay to fine silt) and large, interconnected micropores (2-5 micrometers). The preservation of these micro-interlaminations at burial depths ranging from 2.5 m (modern) to 4.5 km (supermature) indicates that mechanical rearrangement of grains during burial is not sufficient to destroy the original depositional signal. However, increasing maturity (depth of burial) appears to enhance the less-resistant microlaminae at the expense of the resistant microlaminae. Compression of the fine-grained, unimodal resistant microlaminae results in a thickness decrease from approx. 100-2 0 micrometers in the modern sediments to approx. 20 micrometers in the supermature shales. Interconnected micropores within the less-resistant microlaminae display some compaction, but generally maintain their integrity. This preservation exists even in samples with relatively low TOC. Elemental mapping indicates that organic matter is adsorbed at the edges and interstices of grains within the less-resistant microlaminae as well as at the contacts between the microlaminae. The nonuniform response of shale microfabric to changes in thermal regime may influence primary migration within a basin.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.