Abstract
The Datang Formation black rock series deposited in an inter-platform basin are considered to be one of the main source rocks in Southwest China. However, their depositional conditions and organic matter enrichment in such a setting are not well understood. In this study, geochemical characteristics of total organic carbon (TOC), major, trace and rare earth elements (REEs) of thirty black rock series samples from a newly-cut roadside outcrop (Zhenning section) in Southern Guizhou, Southwest China are systematically investigated to examine provenance, paleoredox conditions, paleoclimate, paleoproductivity and sedimentary rate, and further to explore their influence on organic matter enrichment. Results show that TOC contents of the investigated samples range from 0.90% to 2.83%, with an average of 1.45%, with TOC variations being in accord with the stratigraphic cyclicity. These data also indicate that the target black rock series were deposited in a combination of continental island arc and active continental margin tectonic setting that exhibited moderate weathering, oxic/dysoxic oxygen levels, hot and arid to semiarid climate, and a higher sedimentary rate. Multiple paleoredox indicators (e.g., Ni/Co, V/Cr, U/Th and V/(V + Ni) ratios), together with slightly negative Ce anomalies and EFU-EFMo covariation, totally suggest an oxic/dysoxic water environment, which is usually considered unfavorable for organic matter preservation. Average (La/Yb)N ratios (1.41) reflect a higher sedimentary rate, resulting in shortening the exposure time of organic matter in the degradation region of aerobic bacteria under oxic/dysoxic conditions. Of the three factors that could affect organic matter enrichment, including paleoproductivity, paleoredox conditions and sedimentary rate, the relatively high paleoproductivity indicated by the relatively high TOC contents appears to be the dominant factor. In contrast, the other two factors, i.e., paleoredox conditions and the rate of sedimentation play relatively less significant roles. Based on these factors, an enrichment model of organic matter for the black rock series in Southern Guizhou Depression is established. These findings also contribute to constraining the sedimentary environment in such a setting and further deepen our understanding of the potential shale gas resources of the Southern Guizhou Province.
Highlights
In the past few decades, increasing numbers of scholars have demonstrated the applicability of understanding major elements, trace elements (TEs), and rare earth elements (REEs, from La to Lu) data as useful indicators to examine the depositional conditions [1,2,3,4,5,6,7] and to determine the mechanisms controlling the organic matter enrichment in modern and ancient sediments [8,9,10,11]
total organic carbon (TOC) contents for all the 30 samples are presented in Table 1 and Figure 3
TOC contents vary from 0.90% to 2.83%, with an average of 1.45%
Summary
In the past few decades, increasing numbers of scholars have demonstrated the applicability of understanding major elements, trace elements (TEs), and rare earth elements (REEs, from La to Lu) data as useful indicators to examine the depositional conditions (e.g., provenance, paleoredox conditions and paleoclimate) [1,2,3,4,5,6,7] and to determine the mechanisms controlling the organic matter enrichment in modern and ancient sediments [8,9,10,11]. Exceedingly few publications document the organic-rich sediments deposited in an inter-platform basin owing to their limited occurrence in the stratigraphic records [17,18,19]. The Datang Formation black rock series in Southern Guizhou, Southwest. Previous studies suggested that organic-rich black rock series from the Lower Carboniferous. Datang Formation are considered to be deposited in an inter-platform basin [18,20] and be the dominating hydrocarbon source rocks in Southwest China [21,22]. The sedimentary environment of the Datang Formation in Southern Guizhou Depression has been studied by means of lithologic description, petrographic and biostratigraphic approaches [18,20,23,24]. The above-mentioned methods for elucidating sedimentary environments are restricted by the number of well drillings
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