Fine-Scale Geochemical Evaluation of a 350-ft. Long, Complete Stratigraphic Section of the Woodford Shale, Oklahoma U.S.A.

Abstract

The Woodford shale in Oklahoma is a prolific conventional and unconventional hydrocarbon source and reservoir rock. Detailed characterization of the Woodford continues to unlock unique information related to its depositional history/environments, nature of the source material, paleo water chemistry and hydrocarbon potential. A 350 ft. section of a complete Lower, Middle and Upper Woodford outcrop in Southern Oklahoma has been characterized on the basis of 500 samples collected at <0.5 ft. spacing and analyzed for bulk and molecular geochemical properties, chemostratigraphy, lithostratigraphy, and geomechanics. Many significant, preliminary observations have resulted from this study:

(1) Marine shales comprise much of this 350 ft section. It consists mainly of two alternating rock types: chert/siliceous shale and clay-shale beds/laminations.

(2) Average TOC values for the Upper, Middle and Lower Woodford were 8.19, 6.60 and 8.19 wt.% respectively, but individual values ranged from 33.9 to 0.05%. Cherty beds contain relatively less TOC than clay shales. With such a wide range, it is essential to group samples on the basis of their lithology rather than by averaging all analyses together; this will reduce sampling bias and provide more realistic organic richness stratification.

(3) All of the samples were either immature or in the very early oil window. Small variations in Ro% based on calculated Tmax values occur, but these variations more likely resulted from small S2 peaks or weathering, rather than variations in their maturity.

(4) The majority of HI values ranged from 400-800 mg hydrocarbon/g TOC indicating a hydrogen-rich, oil prone kerogen. The OI values in general are relatively low although many samples in the Upper and Middle Woodford have higher OI values which may reflect contemporary weathering; this will be resolved by biomarker analysis.

(5) TOC stratification mimics the fine-scale stratification of the two lithologies and their chemostratigraphy. For example, over one 20ft section TOC alternates at <1ft intervals between high and low values ranging from 1.6 to over 20%. These alternating changes in TOC and HI values are being correlated with changes in specific biomarker parameters, as well as lithologies, as determined by Si/Al ratios (which is a proxy for terrestrial vs. biogenic input). There is a very clear inverse relationship between TOC values and the Si/Al ratio. The frequency of the sampling in this case is approximately 57,000 years.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018