Pingchang Sun

Tectonic and climate control of oil shale deposition in the Upper Cretaceous Qingshankou Formation (Songliao Basin, NE China)

Oil shales were deposited in the Songliao Basin (NE China) during the Upper Cretaceous period, representing excellent hydrocarbon source rocks. High organic matter (OM) contents, a predominance of type-I kerogen, and a low maturity of OM in the oil shales are indicated by bulk geochemical parameters and biomarker data. A major contribution of aquatic organisms and minor inputs from terrigenous land plants to OM input are indicated by n-alkane distribution patterns, composition of steroids, and organic macerals. Strongly reducing bottom water conditions during the deposition of the oil shale sequences are indicated by low pristane/phytane ratios, high C14-aryl-isoprenoid contents, homohopane distribution patterns, and high V/Ni ratios. Enhanced salinity stratification with mesosaline and alkaline bottom waters during deposition of the oil shales are indicated by high gammacerane index values, low MTTC ratios, high β-carotene contents, low TOC/S ratios, and high Sr/Ba ratios. The stratified water column with anoxic conditions in the bottom water enhanced preservation of OM. Moderate input of detrital minerals during the deposition of the oil shale sequences is reflected by titanium concentrations. In this study, environmental conditions in the paleo-lake leading to OM accumulation in the sediments are related to sequence stratigraphy governed by climate and tectonics. The first Member of the Qingshankou Formation (K2qn1) in the Songliao Basin, containing the oil shale sequence, encompasses a third-order sequence that can be divided into three system tracts (transgressive system tract—TST, highstand system tract—HST, and regressive system tract—RST). Enrichment of OM changed from low values during TST-I to high-moderate values during TST-II/III and HST-I/II. Low OM enrichment occurs during RST-I and RST-II. Therefore, the highest enrichment of OM in the sediments is related to stages of mid-late TST and early HST.

Element response to the ancient lake information and its evolution history of argillaceous source rocks in the Lucaogou Formation in Sangonghe area of southern margin of Junggar Basin

With the analysis of the element geochemistry characteristics, the ancient lake information evolution history of the argillaceous source rocks in Lucaogou (芦草沟) Formation in Sangonghe (三工河) area is reconstructed. According to the ancient lake information and total organic matter (TOC) characteristics of argillaceous source rocks, the study section is divided into 6 Subsections. Subsection I mainly developed low-quality source rocks. This is because of the arid climate, high salinity, low lake productivity, unstable preservation conditions in this Subsection. Subsection II mainly developed high-quality source rocks. This is because of the humid climate, low salinity, high lake productivity, stable preservation conditions in this Subsection. Though the paleoclimate was humid and preservation conditions were stable. Lake productivity and the water salinity changed frequently. So Subsection III mainly developed medium-quality source rocks. Because of the humid climate, high lake productivity, medium sedimentary rate and stable preservation conditions, high-quality source rocks were developed in Subsection IV. The preservation conditions were stable, but other ancient lake information changed frequently. Therefore, the quality of the formed source rocks in Subsection V was different. Subsection VI mainly developed high-quality source rocks because of the humid climate, medium sedimentary rate, high lake productivity, low salinity and good preservation conditions. In summary, the ancient lake information parameters and TOC characteristics of each Subsection are different from each other.

Palaeogene sequences with sedimentary characteristics controlling the lacustrine oil shale of the Meihe Basin

As a case study, the Meihe Basin, a typical Cenozoic faulted basin, was divided into and identified as five three-order sequences by utilizing core, well logging, and seismic data, as well as palaeontological and geochemical data. Field measurements of sections, core observations, and a comprehensive analysis revealed that the basin is mainly composed of deposits of alluvial fan, fan delta, lacustrine facies, and gravity flows, and oil shale is mainly developed in semi-deep and deep lacustrine environments. The comprehensive study of the sediment-sequence stratigraphy indicates that Sequence I was formed in the initial rifting stage of the basin, dominated by coarse clastic sediments of alluvial fan and fan delta. Sequence II was formed in the rifting expansion stage of the basin, with more developed sediments of fan delta and lacustrine. Sequence III was formed in the largest expansion stage of the basin, dominated by mudstone of deep lacustrine facies and gravity flow deposits. Sequence IV was formed in the shrinking stage of the basin, dominated by sediments of delta and lacustrine fan. Oil shale are mainly developed in the transgressive system tract (TST) and highstand system tract (HST) of Sequence III (Mudstone Member of lacustrine facies). The lake flooding effect of TST can reduce the decomposition amount of organic matter, increase in organic matter production, and reduce the amount of dilution, thus forming oil shale with a thin consistency but high quality. In the period of the HST, the larger accommodation space and excellent organic matter preservation conditions are conducive to developing stable oil shale with a greater consistency. During the high water level period, however, due to the oxygen brought in by turbidites, the decomposition of organic matter is often increased, resulting in the formation of low-quality oil shale.

Geologic Conditions of Shale Oil and Gas Accumulation in the Southeast Uplift Songliao Basin (Upper Cretaceous, NE China)

Thick dark source rocks were developed in the Qingshankou Formation (Late Cretaceous, Songliao Basin). According to sedimentary characteristics and systematical geochemical tests, the geological conditions of shale oil and gas accumulation are studied. The thick dark mudstone succession (TOC > 2%) is represented by deep lake environment and their thickness could reach 91 m in the center depression, Songliao Basin. Organic matter is mainly composed by type I and II1 kerogen. Vitrinite reflectance (0.5–0.7% Ro) and RockEval parameter Tmax (425–445°C) indicate that the source rock is marginal mature to mature. Abundant brittle minerals (average 51.1%) and the 4.1–14.0% porosity are developed in the mudstone. Based on the SEM and microscope observation, the micron level to nanometer level pores are the main reservoir for shale gas and oil. Through comprehensive analysis of these geological conditions, it is concluded that the lower part in the Qingshankou Formation has the potential to form shale oil and gas reservoirs, and the deep region of the Southeast Uplift near the basin center side is the best favorable zone of shale oil and gas accumulation.

Mineralogy and geochemistry of fine-grained clastic rocks in the Eocene Huadian Basin (NE China): Implications for sediment provenance, paleoclimate and depositional environment

The Huadian Basin is a small fault-controlled basin in northeast China. It is filled by the Eocene Huadian Formation comprising thick lacustrine oil shale-and coal-bearing sediments. Oil shale, mudstone and carbonaceous shale samples have been collected to determine their mineralogical and geochemical (major, trace and rare earth elements) characteristics. These data are used to evaluate sediment provenance as well as paleoclimate and depositional environment. The fine-grained sediments in the Huadian Formation are derived from felsic volcanic rocks and granites, mixed with minor amounts of mafic and sedimentary rocks. Geo-chemical proxies confirm sediment recycling in the source region. Clay mineralogy and indices of chemical alteration suggest that a subtropical warm and humid climate prevailed during deposition of the fine-grained sediments. The data also suggest climatic changes during deposition of the Huadian Formation, from a stable warm and humid climate causing intermediate chemical weathering (Pyrite Member), to a seasonal dry-wet climate (Oil Shale Member), to a stable warmer and more humid climate causing strong chemical weathering (Carbonaceous Shale Member). Based on inorganic proxies, the fine-grained sediments in the Huadian Formation have been deposited in an anoxic fresh-water environment. Only the sediments of the Oil Shale Member reflect fluctuating freshwater and brackish conditions. The Eocene climatic change controlled lake level variations and water chemistry. A brackish and strictly anoxic environment together with a warm and humid climate was beneficial for the formation of high quality oil shale, whereas fresh-water conditions and warm and more humid climate favored peat accumulation.

A comparative geochemistry study of several oil shale-bearing intervals in the Paleogene Huadian Formation, Huadian Basin, Northeast China

The Huadian Basin is an oil shale-bearing basin located in northeastern China. Thirteen oil shale layers deposited in this basin, and the characteristics of oil shale are different among these oil shale layers. Based on the core observation and microscope identification, using the organic and inorganic data from borehole HD3 and outcrops, the formation conditions of different grade oil shale have been evaluated. Based on measuring oil yield (OY in short) of an oil shale to determine its grade, this paper classifies the oil shale as high grade (OY≥10%), medium grade (10%>OY≥5%) and low grade (5%>OY≥3.5%). The high grade oil shale is mainly in brown or dark brown, and the bulk density ranges from 1.59 to 1.81 g/cm3. The results of X-ray diffraction indicate the content of carbonate minerals is 28.0%. The HI (hydrogen index)-Tmax, HI-OI (oxygen index) and S2-TOC (total organic carbon) diagrams indicate the kerogen types are I and II1. The high grade oil shale generally formed in the relatively arid paleoclimate, deposited in the brackish water, dysoxic environment, when the bioprodctivity is extremely high, and the information is mainly from the inorganic parameters diagrams of chemical index of alteration (CIA), Sr/Ba and V/(V+Ni). The medium grade oil shale mainly shows grey-black or black-brown color and the bulk density ranges from 1.87 to 2.08 g/cm3. The average content of carbonate minerals is 16.4%, far less than high grade oil shale and the kerogen type is mainly II1. The inorganic parameters diagrams reflect the medium grade oil shale generally formed in the less humid paleoclimate, deposited in the brackish water, dysoxic to anoxic environment, when the bioproductivity is medium. The color of low grade oil shale is major in grey-black or dark grey and the bulk density ranges from 2.00 to 2.15 g/cm3. The average content of carbonate minerals is sharply decreased and the kerogen type is mainly II1. The inorganic parameters diagrams indicate the low grade oil shale generally formed in the relatively humid paleoclimate, deposited in the freshwater to brackish water, anoxic environment, when the bioproductivity is relatively low. Comprehensive study suggest the increasing precipitation caused by a relatively humid paleoclimate resulted in decreasing oxygen concentration and salinity in the bottom water, restrained the salinity stratification, and tended to form the low grade oil shale. The strong evaporation leading to relatively arid paleoclimate resulted in increasing oxygen concentration and salinity in the bottom water, and promoted the salinity stratification. The area with well preservation conditions caused by the relative high sedimentary rate and salinity stratification are favorable to high grade oil shale deposit. In summary, the bioproductivity and the paleoclimate are the main factors controlling the oil shale grade in the Huadian Basin, when the bioproductivity is relatively high and the paleoclimate is relatively arid, it is prone to form the high grade oil shale. However the low grade oil shale developed in the low bioproductivity and relatively humid paleoclimate.

Deposition of Oil Shale in the Fushun Basin, Eocene, N-E China

The Fushun Basin (NE China) is located along the Dunhua-Mishan fault zone. This lacustrine strike-slip basin hosts the largest open pit coal and oil shale mine in Asia. Oil shales overlying sub-bituminous coal occur within the middle Eocene Jijuntun Formation. The main aim of the study was to reconstruct the depositional environment of the oil shales. Furthermore, the source rock potential was evaluated. A total of 50 samples from the West Open Pit mine were studied using Rock Eval pyrolysis, Leco, biomarker and maceral analysis. The results show that the oil shale was deposited in a tectonically controlled freshwater lake, which originated after drowning of a peat. Organic matter rich sediments with TOC contents up to 23.6 wt.% containing kerogen type I (formed by bacteria and algae) were deposited under anoxic conditions in the upper part of the Jijuntun Formation, probably caused by temperature stratification of the water column. We speculate that this water column stratification was supported by increasing depth of the lake during oil shale deposition. Landplants and aquatic organisms are abundant in the lower part of the Jijuntun Formation. The amount of landplants decreases upwards.