Formation and Characteristics of the Cryogenian High-Quality Source Rock, South China

Abstract

Following the breakup of Rodinia supercontinent and globally distributed Sturtian glaciations, a well-preserved source rock deposited as a postglacial warm interval develops in the mid-Cryogenian Datangpo formation, South China. Datangpo formation constrained by zircon U-Pb ages of 664Ma~654Ma conformably lies between two glacial diamictites timely equivalent to the Sturtian and Marinoan, respectively. The source rock develops in the lower member of Datangpo formation and upwards consists of manganese-bearing carbonaceous shale, manganese carbonate (rhodochrosites) and black carbonaceous shale with a total thickness more than 50m. Geochemical results indicate it a high-quality source rock with TOC values ranging between 1.22%and 8.5%(ave=2.8%, n=122) and kerogen types of I and II1. Vitrinite reflectance (Ro) of 2.0%~2.8% indicates overmatured dry-gas producing stage. Compared with contemporaneous deposits found in East Greenland and Svalbard, the studied source rock proves to be more organic-rich and lithologically different with deposition of manganese carbonate. Elemental and isotopic analysis favors two depositional stages of this source rock. The first stage characterized by organic-rich manganese-bearing deposition exhibits variable CIA values of 52-68, abnormally positive δ34S of 43.7‰~62.6‰and lighter δ13C values of -5.79‰~-10.84‰,indicating oscillatory paleoclimate favorable for Mn-oxides/hydroxides formation around the redox line and essential euxinic environment for the reaction between Mn-oxides/hydroxides and descending micro-organisms to form manganese-bearing deposits. The second stage featured by thicker black carbonaceous shale develops in a relatively warm and more stable redox-stratified water column indicated by Fe speciation, molybdenum isotopes and elemental analysis. Data from 56 outcrops shows thickness of the source rock varies from 0.5~55m, with the depocenter occurring along NE and NNE trending grabens. This indicates good development of Cryogenian source rock mostly occurs in rifted areas and favorable accumulations might be found in the adjacent stable paleohighs. With the discovery of Anyue supergiant gas field with trillions of cubic meters resource in the Ediacaran-Cambrian in Sichuan basin, South China, this study might provide more available data for deep and ultra-deep petroleum exploration towards more ancient strata.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019