Depositional Controls On Mudstone Lithofacies In A Basinal Setting: Implications for the Delivery of Sedimentary Organic Matter

Abstract

Deep-water mudstones from ancient epicontinental settings are significant repositories for organic matter, but the detailed temporal variations of, and controls on, the abundance and type of organic matter (OM) is little studied. Using micropetrographic and geochemical data from late Mississippian mudstones of the Widmerpool Gulf, UK, the processes that delivered fine-grained sediment to this basin during a glacioeustatic sea-level cycle are interpreted from detailed lithofacies analysis. Seven primary lithofacies are identified from core, which show specific and systematic variations in total organic carbon (TOC) content and bulk carbon isotope composition of organic material (d 13 C org ). During sea-level highstands, thin-bedded carbonate-bearing mudstones are the dominant facies deposited, contain up to 6.6% TOC (average 4.6 ± 1.3%), and have mean d 13 C org of 228.5 ± 0.9%. During phases of lower sea level, thin-bedded silt-bearing clay-rich mudstones with up to 4.1% TOC (average 2.3 ± 0.8%; mean d 13 C org : 228.2 ± 1.0%) were interbedded with more organic-lean graded silt-bearing mudstones and sandbearing silt-rich mudstones (average TOC: 1.7 ± 0.6%) derived from turbidity currents. The latter (mean d 13 C org : 226.2 ± 0.7%) are closely linked to significant proportions of terrestrial plant material, while some rare plant debris-and sand-bearing mudstones produced from debris flows have more than 7.0% TOC and d 13 C org $ 226.0%. The d 13 C values of wood fragments ranged from 227.1% to 224.0% and therefore the d 13 C org is interpreted as a function of the ratio of marine and terrestrial organic matter. More negative values in the carbonate-bearing and the clay-rich mudstones indicate marine planktonic algae whereas the least negative values reflect greater contribution of terrestrial plant material. The data suggest that the marine conditions prevailed and supported marine planktonic algae throughout different sea-level stages. Marine OM was delivered to the sea floor by continuous hemipelagic settling whereas terrestrial OM was delivered by sediment density flows. Variations in bioproductivity and dilution by siliciclastics influenced the burial rate of marine OM. Organic-rich mudstones preserved in these marine basins are potential hydrocarbon source rocks, especially as unconventional (shale gas) reservoirs. Detailed microtextural and compositional analysis coupled with rigorous geochemical parameters as used in this study are important for the understanding of the source-rock potential of basinal mudstones, and of fine-grained organic-rich sediments in general.

On the primary and secondary petroleum generating characteristics of the Bowland Shale, northern England

“…Sedimentological, organic petrological and stable carbon isotope studies (Armstrong et al 1997;Fraser & Gawthorpe 2003;Könitzer et al 2014) have shown that the upper Bowland Shale in this study is marine shale from predominantly hemi-pelagic deposition and organic matter is derived from planktonic phytoclasts. These attributes are better represented by the isolated kerogen pyrolysis data (Rock-Eval, PyGC and bulk kinetic results) than by the equivalent whole-rock data.…”

“…All three samples have δ 13 C org values between −28.0 and −28.4‰, indicating that the kerogen is derived from marine planktonic algae, so the three samples are representative of the thick hemi-pelagic mudstones, the dominant lithofacies in the succession at Carsington ( Fig. 2; Könitzer et al 2014).…”

“…Both hemi-pelagic marine shale and pro-deltaic turbiditesdeposited in deep-basin waterwere recognized using micropetrography and TOC-bulk δ 13 C org data (Könitzer et al 2014). Thick intervals of marine Bowland Shale are relatively homogeneous on a metre scale, whereas intervals of interbedded shales and turbiditic sandstones are relatively more heterogeneous, as revealed by petrophysical (Hough & Vane 2014), geochemical (Gross et al 2014) and lithological (Könitzer et al 2014) properties. Three unweathered core shale samples as well as their kerogen concentrates from the marine part of the well were tested in this study.…”

“…Biomarkers and stable carbon isotopes have established likely precursor biota of the organic matter (Ewbank et al 1993;Armstrong et al 1997). Also, Könitzer et al (2014) have used total organic carbon (TOC) and carbon isotope composition to differentiate various depositional environments vertically.…”

Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

“…paleoproductivity and bottom water redox conditions in a periodically stratified marine basin [24]. Depositional processes, productivity, and bottom water redox conditions have a profound control on the basin-wide distribution of lithofacies as well as the amount and quality of organic matter [25][26][27][28][29][30][31][32][33]. The Sichuan Basin developed on the Precambrian metamorphic basement of the upper Yangtze craton [34].…”