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Black Shales of Paleogene Subathu Formation, NW Himalaya: A Promising Resource of Organic Matter

Neelam, Siva Siddaiah 1
(1)Geochemistry, Wadia Institute of Himalayan Geology, Dehra Dun, India.

Black shales rich in organic matter (2.6 -11.8 wt% TOC) occur in the early Paleogene part of the Himalayan foreland basin coinciding with the India-Eurasia initial collision timing. This paper presents mineralogy and geochemistry of black shales of the shallow marine Subathu Formation that are important in evaluating their energy potential and in selecting a suitable processing technology.

The black shales of the Subathu Formation are ~20-35m thick, exposed throughout the NW Himalaya. They are massive to finely laminated and consists of kaolinite, illite, quartz, calcite, feldspar, berthierine, rutile and kerogen. Pyrite occurs as disseminations. They consists dominantly of SiO2, Al2O3, Fe2O3 and CaO followed by TiO2 and K2O, while MgO, Na2O, P2O5 and MnO have a concentration of <1 wt%. The most abundant trace elements (in ppm) are Zr (77-648 ppm), Sr (54-285), Ba (12-358), Zn (25-151), V (92-364) and Rb (0.5-126), other elements occur in <100 ppm. CaCO3 and TOC contents in the black shales show an inverse relationship. A sharp increase in TOC to a maximum of 11 wt % is found at the base of the unit and gradually decreases upwards to 2.5 wt %, while CaCO3 varies from 1-14 wt %.

The total REE content of black shales ranges from 177.35 to 470.61 ppm, which are higher than that in limestones(ΣREE= 92.31 ppm). Chondrite-normalized REE patterns are similar for all the samples, displaying sloping LREE (Lan/Smn = 3.1-4.46), relatively flat HREE trends, with a minor positive Ce anomaly (1.1) and with a distinct negative Eu anomaly (0.51-0.69) excepting one sample. Shale-like REE patterns of black shales and limestones indicate that REEs were derived from a terrigenous source. REE contents in black shales are controlled by clays and partly by organic constituents.

Black shales of the Subathu Formation were deposited under anoxic conditions during low sea level, which is reflected by the occurrence of shallow-marine and non-marine sediments in the sequence and the preservation of organic matter indicates deposition under dysoxic conditions associated with increased flux of organic matter. The geologic setting, mineralogy and characteristic geochemistry of the early Paleogene organic-rich carbonate succession in the Himalayan basin suggests it’s potential as an organic-carbon source. Further systematic multi-disciplinary research is warranted to get insights in to the relation between collision, climate, provenance and preservation of energy resource.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.