Main Controlling Factors and Models of Organic Matter Accumulation in Lower Carboniferous Dawuba Formation Shale in Southern Guizhou, China.

A set of high-quality marine facies organic-rich shales developed in the Lower Carboniferous Dawuba Formation, which is considered to be the main target of shale gas exploration and development in Guizhou Province. In this paper, 53 samples from Well ZY1 are selected, and the core observation data, field-emission scanning electron microscopy (FE-SEM) images, and geochemical data of these samples are analyzed. On the basis of these data, the main influencing factors of organic matter enrichment in the Dawuba Formation shale were identified and an organic matter accumulation model was established. The results show that total organic carbon (TOC) values of the Dawuba Formation in the ZY1 well vary between 1.97 and 4.11%, with high values appearing at the depths of 2796-2814 m (3.00-4.11) and 2877-2894 m (1.97-3.49). The redox-sensitive element enrichments are generally low, indicating that these samples were deposited under oxic-suboxic conditions. The micronutrients (Zn, Cu, and Ni), biological Ba (Ba), and P/Al also show low values, indicating low primary productivity. The chemical index of alteration (CIA) and terrigenous clastic input index (Ti/Al) showed two obvious high-value zones, indicating that shale in the study area was affected by terrigenous inputs. Similarly, the calculation results show that Fe/Mn and Rb/K values have two abnormal data segments at the same depth. The anomaly of these data at the same depth section further suggests that the shale was affected by terrigenous input during deposition. Moreover, the terrigenous input reaches the maximum in the above TOC high-value region, and it is inferred by combining with the core observation results that the gravity flow occurs in this depth. The carbon isotope of kerogen (δC) ranges from -26.84 to -24.36%, indicating that the source of organic matter is likely to be terrestrial plants. This is further supported by the widespread presence of filamentous organic matter using FE-SEM, despite the low productivity and poor preservation conditions during deposition of the Dawuba Formation; the enhanced terrigenous input may have provided additional sources of organic matter for the Dawuba shale.