Grain Size Distribution of Organic Matter and Pyrite in Alum Shales Characterized by TIMA and Its Paleo-environmental Significance

Abstract

BACKGROUND Alum shale (Middle Cambrian to Lower Ordovician) is a set of significant marine source rock in Northern Europe, and the maturity is from immature to over mature. Because of high maturities of the Lower-Paleozoic marine source rocks in China, the immature-early mature Alum shales are important samples for the comparative studies of hydrocarbon generation potential. Thus, the analyses of organic matter, mineral composition and sedimentary characteristics of the Alum shale with low maturity, are the fundamental for the comparative studies on the Lower Paleozoic marine source rocks at home and abroad. OBJECTIVES To uncover the mineral and organic matter compositions, and the sedimentary characteristics of Alum shales. METHODS Based on the organic carbon content (TOC) and organic petrological observations, the integrated mineral analysis (TIMA) technology was used to scan the sample. Scan parameters were refined to obtain the mineral composition, content and particle size distribution of shale. RESULTS The TOC and TRS contents of studied Alum shale (%Rb=0.3) were 11.16%-12.24% and 4.30%-5.31%, respectively. The maceral compositions included the lamalginite from planktonic algae degradation and vitrinite-like macerals from benthic algae. Solid bitumen filled in the fractures and pores. The organic matter (OM) and pyrite (Py) contents acquired from TIMA scanning analyses were 9.79%-10.64% and 4.17%-4.49%, respectively. The OM/Py ratios ranged from 2.18 to 2.55, similar to the C/S ratios from Rock-eval pyrolysis. Grain sizes of OM were 0.9-27.0μm, and 80% grains had sizes of 1.2-5.5μm. Grain size of Py distributed from 0.9 to 17.0μm, and the proportion of grains with the size of lower than 0.5μm was higher than 78%, indicating an oxygen-lacking or sulfurized environment. CONCLUSIONS A combination of C/S ratio, and characteristics of organic petrology and Py grain size distribution indicates the Alum shale was deposited in a closed or sulfurized water system. A new technical support for the research of source rocks (including shale) in the field of petroleum geology is provided through the methods used and discussed in this study.

Keywords

• tima technique
• alum shale
• mineral composition
• organic matter
• pyrite
• grain size distribution
• sedimentary environment