Algal-Microbial Community, Paleoenvironment, and Shale Oil Potential of Lacustrine Organic-Rich Shale in the Upper Cretaceous Nenjiang Formation of the Southern Central Depression, Songliao Basin (NE China)

Abstract

The Songliao Basin is the largest oilfield in China and across southeast Asia. The Upper Cretaceous Qingshankou and Nenjiang Formations in the basin comprise the source rocks for conventional oil and gas, as well as the primary exploration target for shale oil. Although the lacustrine organic-rich shale (LORS) in the first member of Nenjiang Formation (K 2 n 1) from the southern Central Depression is characterized by high organic matter (OM) abundance and the dominance of oil-prone kerogen, a study on the algal-microbial community, paleoenvironment, and shale oil potential of them is still lacking. Here, we address these issues based on the integration of bulk geochemical, organic petrographic, biomarker, scanning electron microscopic, and nitrogen gas adsorption analyses. The algal-microbial community was mainly composed of red algae, green algae, and dinoflagellates, together with bacteria and a minor proportion of land plants in the paleolake, which might be favorable for OM productivity. Saline to brackish and anoxic to dysoxic conditions were present in the paleolake, which were conducive to OM preservation. Both the favorable OM productivity and preservation conditions resulted in deposition of the K 2 n 1 LORS, which was characterized by high total organic carbon (TOC) contents and the dominance of oil-prone kerogen (type I). However, limited hydrocarbon generation potential of shale oil was present in the K 2 n 1 LORS, because of the relatively low maturity (early oil window maturity). Nano- to micrometer pores, including interparticle pores, intracrystalline pores, dissolution pores, and OM pores, formed two types of pore structures and pore size distributions in the K 2 n 1 LORS. In comparison with LORS from the first member of Qingshankou Formation (K 2 qn 1) in the study area, the K 2 n 1 LORS displays higher TOC and HI values, lower OSI and T max values, and a higher Brunauer–Emmett–Teller surface area and Barrett–Joyner–Halenda pore volume. The favorable area of shale oil in the K 2 n 1 LORS is proposed based on the integration of OM abundance, type, and maturity. In addition, the K 2 n 1 LORS buried with a depth of <1000 m may have great oil shale potential, which is suitable for in situ conversion processing. However, further research on the planar distributions of oil yield, shale thickness, underground water, laminar fractures, and OM pores of the K 2 n 1 LORS are needed. This work is not only beneficial for paleoecological reconstruction in the Songliao Basin during the K 2 n 1, but also provide guidance for regional unconventional hydrocarbon exploration in the future.

Origin of organic matter and depositional characteristics of early Cambrian Niutitang Formation from South China: New insights using molecular fossils

“…Hopanoids have been observed in the Niutitang Formation, maximizing at C 30 hopanes (Figure 3g,h). Hopanes are thought to originate from bacteriohopanols that usually occur in eubacteria, more precisely in bacteriohopane polyols (Liu et al, 2021; Ourisson et al, 1979; Peters et al, 2005). Rearranged hopanes (C 30 *: 17α‐diahopane) and neohopanes have been observed in bitumen extract of the Niutitang Formation and are considered to be formed by the diagenetic alterations of the regular hopanes, while their biological precursor has remained unclear (Seifert & Moldowan, 1978).…”

Pore types, genesis, and evolution model of lacustrine oil-prone shale: a case study of the Cretaceous Qingshankou Formation, Songliao Basin, NE China

“…Thus, feldspar and carbonate minerals underwent continuous dissolution. However, the water rock reaction was not significant; thus, the pores formed by dissolution were also limited because the shale was subjected to continuous high compaction and cementation 26 , 27 . Owing to the continuous hydrocarbon generation and expulsion, most organic acids are constantly discharged from shale reservoirs 28 , and acidic substances in pore fluids are consumed during dissolution.…”

Comprehensive Outlook into Critical Roles of Pressure, Volume, and Temperature (PVT) and Phase Behavior on the Exploration and Development of Shale Oil

“…In the same stage of maturity, due to different types of organic matter within the source rock, the generated hydrocarbon components will also vary. As a result, the phase behavior of oil and gas will show differences . When the burial depth and temperature of the kerogen increase and the evolution degree continues to rise, the organic polymer components are gradually transformed into smaller molecules.…”

“…As a result, the phase behavior of oil and gas will show differences. 153 When the burial depth and temperature of the kerogen increase and the evolution degree continues to rise, the organic polymer components are gradually transformed into smaller molecules. Therefore, the hydrocarbon generation process can be defined as a gradual accumulation of light hydrocarbons to subsequent oil, condensates, and wet and dry gas.…”