Search

Ordos Basin, a large lacustrine basin in the Triassic, possesses amounts of conventional and unconventional petroleum. In the basin, the Chang 7 shales have been studied extensively. Recent studies suggested that the crude oils accumulated in the Triassic Chang 9–8 should be mainly sourced by the Chang 9 shales [including the delta plain shale (DPS), delta front shale (DFS), and deep lake shale (DLS)]. The source rock (SR) potential, hydrocarbon generation (HG), and hydrocarbon expulsion (HE) characters of the Chang 9 shales should be analysed to make a comprehensive source rock evaluation. The shales are evaluated as excellent source rock potential, and the DLS is evaluated as the best potential. The HG–HE of the shales are analysed by utilizing two (closed and open systems) pyrolysis experiments. The DLS activation energy (Ea) shows the narrowest distribution and has the lowest average value of 219.00 kJ/mol. The HG onsets and total amounts of the DFS, DPS, and DLS have been determined (165 Ma and 200 × 103 t/km2, 157 Ma and 305 × 103 t/km2, 154 Ma and 430 × 103 t/km2), and the peak generation of the shales happened at 125–117 Ma, 127–100 Ma, and 125–100 Ma, respectively. The Chang 9 DFS, DPS, and DLS have reached the HE threshold at depths of 2,250, 2,500, and 2050 m, respectively, indicating large amounts of hydrocarbons have been expelled into the Lower Yanchang Formation (LYF). The tight sandstone oil reservoirs (TSORs) are mainly located in the areas where the organic‐rich shales are developed, especially in the northeastern study area. The areas between high‐value zones of source rock properties (e.g., thickness, Ro, total organic carbon, HG–HE intensity) are the sweet spots. [ABSTRACT FROM AUTHOR]

The Ordos Basin, a large mainly non‐marine basin, is an important petroliferous basin in northern China. The Triassic Chang 9 shales, including the delta plain shale (DPS), delta front shale (DFS), and deep lake shale (DLS), are proven to be the main source rocks in the lower Yanchang Formation (LYF). This study investigates the source rock potential, palaeoenvironment, and organic matter (OM) source. According to the OM thermal maturity, abundance, and type, the shales have excellent source rock potential. The DLS has the highest quality with average TOC of 7.08%, S1 + S2 of 14.41 mg HC/g rock, and Type II1–II2. According to correlations of Rb/Sr versus Sr/Cu, the shales were mainly deposited in arid conditions. Based on the major element (S), trace elements (V, Ni, Co, Cr, U, and Th), rare earth element (Ce) concentrations, and biomarkers (Pr, Ph, dibenzofuran, fluorene, and dibenzothiophene), redox conditions for the DPS, DFS, and DLS were oxidizing, sub‐oxidizing to sub‐reducing, and reducing, respectively. The palaeosalinity was fresh‐ to brackish‐water, ascertained by the trace elements (Sr and Ba) and biomarkers (gammacerane). The water stratification was controlled by the temperature changes and the palaeosalinity differences, and the maximum water depth was ~100 m during the Chang 9 deposition. Aquatic microorganisms (e.g., algae, bacteria, and phytoplankton) were the main organic matter sources for the shales, mixed together with terrigenous plants in DPS and DFS. The DLS has the most aquatic microorganisms and has the least higher plant fragments, supported by the higher TOC (up to 14%), lowest vitrinite contents (Avg. = 14.1%), and highest liptinite contents (Avg. = 59.5%). Laterally, the DPS, DFS, and DLS were distributed successively from the lake margin to the depocentre. Vertically, the DLS, DFS, and DPS were developed successively from the bottom lake to the upper lake. The DLS was mainly formed in an environment of high‐level sapropelinite and favourable preservation conditions, and the DPS was mainly formed in an environment of high‐level terrigenous inputs and poor preservation conditions, while the DFS was mainly formed in an environment of medium‐level of sapropelinite productivity and terrigenous inputs and fair preservation conditions. The DPS and DFS were formed during both the early stage (shallow lake) and the late stage (deep lake) of the Chang 9 deposition, while the DLS was only developed in the deep lake during the late Chang 9 deposition. The high‐level sapropelinite production and favourable preservation controlled the formation of the organic‐rich shales, especially for the organic‐rich DLS. [ABSTRACT FROM AUTHOR]

The deep formations (underlying strata of the 3rd Member of Quantou Formation, Upper Cretaceous) in Songliao Basin possess a large natural gas potential, with a proven reserve of more than 200 × 109 m3 (706.3bcf). However, due to deep burial depth, multiple tectonic evolutions, and sets of source rocks, complex gas accumulations and different gas reservoir types developed, which may seriously restrict the natural gas exploration and target selection. This study focuses on the hydrocarbon generation, porosity evolution, and hydrocarbon accumulation processes of the tight sandstone gas (TSG) reservoir in the Cretaceous Shahezi Formation. 1) The timing and cumulative amounts of hydrocarbon generation were ascertained, with the Huoshiling being 143–10 Ma and 108.00 mg/g TOC and the Shahezi being 122–44 Ma and 30.00 mg/g TOC, respectively. 2) The mean original porosity of the Shahezi sandstone was 23.11%, and experienced the porosity decreasing of compaction (avg. = 12.57%) and cementation (avg. = 7.07%), and porosity increasing of dissolution (avg. = 3.25%), and the average present porosity is 6.72%. The sandstone densification (porosity <10%) occurred in the late period of Early Cretaceous (~115–105 Ma). 3) Four main stages of hydrocarbon accumulation in the K 1 sh occurred: I) Early oil accumulation stage (128–114 Ma)，corresponding to the 1st episode of charging; II) Middle oil and gas accumulation stage (111–90 Ma), corresponding to the 2nd episode of charging; III) Late gas accumulation stage (77–67 Ma), corresponding to the 3rd episode of charging; and IV) Adjustment stage (63 Ma–present), corresponding to the uplift stage. The 2nd and 3rd stages were the main gas accumulation stages. 4) Two types of tight sandstone gas reservoirs developed in the K 1 sh: I) tight conventional gas, and II) tight deep gas. The tight deep gas reservoirs, with favorable potentials, are the main exploration targets in the Shahezi Formation of Songliao Basin. • Two sets of coal-bearing source rocks generating gas. • Shahezi sandstone densification occurred in Early Cretaceous. • Three episodes of hydrocarbon charging occurred. • Four main stages of hydrocarbon accumulation occurred. • Two types of tight sandstone gas reservoirs developed. [ABSTRACT FROM AUTHOR]

The Songliao Basin is the most productive petroliferous lacustrine basin in NE China, and numerous large gas fields with large proven reserves occur in its deep formations. However, considerable challenges remain: (1) the origins and genetic types of the natural gases are controversial; (2) the gas-source correlations are poorly studied; and (3) the migration distance is vague. In this study, these problems are addressed by the study of the gas compositions, light hydrocarbons, and stable hydrogen and carbon isotopes. The gases are predominantly of organic and thermogenic origins. The Huoshiling (J3h) and Shahezi (K1sh) gases are mainly mixtures of coal-derived and oil-associated gases and the mixed-sources of primary kerogen degradation and secondary oil cracking, while the Yingcheng (K1yc) gases are mainly coal-derived gases and predominantly derived from primary kerogen degradation. The gases in different sags are derived from the source rocks developed in the same sags where the gases accumulated, characterized by the proximal-source accumulation. Vertically, the gases in the J3h and K1sh are predominantly sourced by the proximal J3h and K1sh mudstones, while the gases in the K1yc are mainly derived from either the J3h or the K1sh source rocks, suggesting the gas migration with short distances. [ABSTRACT FROM AUTHOR]

The superimposed foreland basins mainly develop in central-western China, namely the Himalayan foreland basins superimposed on the Indo-China foreland basins. Previous research on hydrocarbon migration predominantly focused on the typical foreland basins and the fore-deep depression of superimposed foreland basins. As a result of multi-superimposition, large hydrocarbon potentials in the slope zones are evaluated, and the hydrocarbon migration shows significant differences with those in the typical foreland basins. Moreover, the hydrocarbon migration models in the slope zone of superimposed foreland basins are poor analyzed. The Chepaizi High in the South Junggar Basin, a typical slope of superimposed foreland basin, has been taken into consideration. Several points have been ascertained. 1) Multistage charging and readjustment events should be the predominant characters. 2) The combination of multi-type sand-bodies, multistage faults and unconformities have formed step- or "Z"-shaped migration pathways spatially. 3) Hydrocarbons migrated from the areas of high oil potential in the fore-deep depression to those of low oil potential in the slope. 4) The migration models of the J 1 b, K 1 q, N 1 s 1 , and N 1 s 2 have been established. Hydrocarbon reservoirs in the slope zones show the shallow accumulation from deep/distal sources with multiple reservoirs vertically and multi-charging and -readjustments chronologically. 5) The areas of (Wells SM3, C53-C90, and CF2), (Wells SM5, C80, and CF13), and (Wells K8, HG4, SM4, and CF2) can be the favorable targets for the J 1 b, K 1 q, and N 1 s, respectively. • Z-shaped and step-shaped composite hydrocarbon migration pathways. • High oil potential located in the fore-deep depression. • Hydrocarbons migrated from the fore-deep depression to the slope. • Multistage migration histories and multistage readjustments in the slope. • Hydrocarbon charged from deep sources and over long distances. [ABSTRACT FROM AUTHOR]