The Nature and Classification of Organic-Associated Pores Based on In-situ Organic Petrology Through Comparative Study on Marine, Transitional, and Lacustrine Gas Shales in Typical Areas, China

Abstract

Abstract

Analyses of organic-rich shales from well cores that penetrated the marine, transitional and lacustrine shales developed in typical areas of China, were performed to evaluate the nature of organic-associated pores based on the comprehensive organic petrographic and SEM analyses.

At least five types of organic macerals (alginate, bitumen, mineral bitumen groundmass, liptinite and vitrinite), four distinct OM occurrence pattern based on its correlation with surrounding minerals (free OM, mineral-adhered OM, clay-combined OM, intergrowth OM), and four subtypes of organic-associated pores based on genetic mechanism (hydrocarbon-bubble pore, hydrocarbon-dissolution pore, fossil-skeleton pore, OM-shrinkage pore) were identified in backscattered-image analyses.

Mineral-adhered OM and clay-combined OM nearly corresponding to bituminite and mineral-bitumenous groundmass (MBG) respectively, are the primary OM occurrence patterns and usually exit in lacustrine shales in low maturation and marine shales in high maturation respectively, which is in contrast with the free OM generally as the particulate material with bio-texture (e.g. alginate, liptinite, vitrinite, fusinite and detritus of them) but mainly distributed in transitional shales for its abundance of vitrinite.

Hydrocarbon-bubble pore is the most common and significant organic-associated pore type, which is generated by gas breakthrough out of the OM surface under the expansion force in gas window and widely developed in marine shlales rich in free algnite and clay-combined OM. Hydrocarbon-dissolution pore is essentially the mineral-hosted pores accompanied with stong organic-acid corrosion caused by mineral-adhered OM or bitumen-coating in oil-window, and hereby is mostly developed in most of lacustrine shlales but in small quantity. OM-shrinkage pore typically occurs along the conjunct edge between organic and inorganic constituents and is usually developed in the transitional shales due to OM occurrence. Fossil-skeleton pore usually distributed in organic detritus under certain pattern with specific geometric shape is less developed in all of the shales.

In addition, a further study on the correlation between various OM occurrences and the evolution of organic-associated pores should be made, since the various associative relations between the organic matter and clay minerals could cause difference in their thermostability and further affected the hydrocarbon generation.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016