Primary Migration of Petroleum in Organic Rich Shale: Observations From Eagle Ford Horizontal Core, South-West Texas

Abstract

A 168 ft. long Eagle Ford horizontal core interval provides a perfect sample set to better understand primary petroleum migration through source rocks regardless of thermal maturity and organic matter type difference. A series of geochemical analysis, including SARA quantification and quantitative C15+ saturated hydrocarbon composition analysis, were conducted on 6 pucks along the horizontal core. Twenty‐one samples were selected from layers of differing mineralogical composition within the pucks. Carbonate‐rich facies have about 40% higher oil saturation than clay mineral‐rich facies. High‐resolution spectral gamma logs were used to examine variations in oil saturation with lithology. The corrected borehole gamma curve (CGR) displays an inverse relationship between clay mineral‐rich intervals and oil saturation. Low CGR indicates carbonate‐rich intervals whereas high CGR indicates clay mineral‐rich. TOC content for all samples ranges from 3.3% to 5.4%, with an average value of 4.4%. Calcite content ranges 41% to 64%, with an average of 51%. Primary oil migration in the core was identified from variations in the amount of extractable organic matter (EOM) in different facies. Layers showed greater EOM/TOC values compare to average values (350mg/g), indicating oil migrated into these layers from layers with lower EOM/TOC values (< 350mg/g). Vertical oil migration occurred from organic‐rich layers to adjacent calcite‐rich layers, as indicated by higher concentrations of C13‐C16 light hydrocarbon fractions (> 100ug/g saturated hydrocarbon per rock) in calcite rich layers. The results of hydrocarbon expulsion during primary migration are only controlled by the spatial continuous distribution of calcite‐rich facies in the shale. Two different types of calcite‐rich layers are present which vary in fabric and origin. The locally distributed calcite‐rich layer might develop as a reservoir barrier. The more continuous regionally distributed calcite layers are favorable for fracture or migration pathway development. Oil generated from their adjacent organic‐rich layers quickly expel and charge in regionally distributed calcite layers through this migration pathway. Our studies provide important insights to the controls of lithology variations on primary migration of petroleum in organic rich shales at the core scale and imply similar controls on migration and saturation at larger scales.

AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019