Organic Petrology Investigation of the Upper Permian Lacustrine Lucaogou Formation, Northwest China: Similarities and Differences as a Function of Lake Type

Abstract

The organic petrography of shale samples (n=20) from the Upper Permian Lucaogou Formation, NW China, was used to evaluate the nature of organic matter (OM) in the context of an evolving lake system. The samples were widely spaced over a ~200 m cored section, and included 1) organic-rich intervals from the underfilled lake portion (lower stratigraphic interval) of the formation, and 2) organic-lean intervals, including siltstones, in the overlying balanced-filled lake portion. OM is dominated by moderate to strongly fluorescent amorphous algal(?) material that occurs in a continuum from lamellar stringers, 10–20 μm thick, up to >1 mm in length (microbial mat?) to a finely-disseminated organic groundmass intimately intermixed with mineral matrix. The organic groundmass may derive from multiple pathways, including: 1) original submicroscopic OM (e.g., algae), 2) mechanical and chemical breakdown of larger unicellular algal bodies (telalginite) and lamalginite (microbial mat?), and 3) bacterial biomass products. A unicellular prasinophyte green alga(?), similar to Tasmanites in marine rocks, is present as discrete flattened discs 50–100 μm in diameter. The abundance of Type I OM is consistent with a lacustrine origin for the Lucaogou, and strong fluorescence indicates that OM has undergone limited thermal maturation. Type III OM including vitrinite and inertinite also is abundant. Some vitrinite with remnant structure and reddish internal reflections has low-moderate fluorescence, indicating an atypical H-rich composition. Solid bitumen fills pores and occurs throughout the entire stratigraphic interval in a wide range of morphology, reflectance, and fluorescence intensity. A discrete bitumen population with low reflectance (0.26–0.36% +/− 0.04–0.09) is interpreted as an early-hydrocarbon generation product. Vitrinite is difficult to conclusively discriminate from high-reflecting solid bitumen and low-reflecting inertinite because their reflectance values overlap with vitrinite. Nevertheless, vitrinite reflectance values were relatively low at 0.47–0.58% (+/− 0.04–0.07), and although consistent with some fluorescence data, the measured Ro values are low relative to other thermal maturity indicators. Qualitatively, microbial mat(?) is present or better preserved in the underfilled portion of the lake, whereas other macerals generally are present consistently throughout the section.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015