In China, pre-Quaternary solid potash deposit has only been discovered in the Simao Basin, and the Lower Cretaceous Mengyejing (MYJ) Formation (Fm.) is the productive layer of potash deposit. In this study, we investigated the clay conglomerates which are distributed in upper and lower members of the potash-bearing salt rock layer. We analyzed the relative contents of major elements (Al2O3, Fe2OT 3, MgO, CaO, Na2O, K2O) and trace elements (B, Ba, Co, Cr, Cu, Ga, Mn, Ni, Rb, Sr, V, Zn, Zr) in the samples. The results show that MgO and CaO in the major elements are rich relative to Post Archean Australian Shale (PAAS), whose average enrichment factor values of the MgO (EFMgO) is 2.61 and CaO (EFCaO) is 4.57, and the others major elements are relatively minor; trace elements (B, Ga, Mn, Zr) are rich relative to PAAS, and the others trace elements are minor relative to PAAS. The study of paleogeographic conditions using various parameters shows that the paleoclimate is generally dry and hot during the period of clay conglomerate deposition, but it was warm and humid in certain periods; the main sedimentary environment is weak oxidation condition with strong oxidation conditions in individual periods; the average value of paleosalinity is ~21‰, and the highest is no more than ~92‰. The significance of the paleogeographic characteristics of MYJ Fm. to potash mineralization are as follows: (1) they indicates that the clay conglomerates of MYJ Fm. are not clastic sediments in brine formed by seawater, because the paleosalinity of clay conglomerates deposition period is obviously lower than that of seawater; (2) MYJ potassic salt ore is not formed by evaporation and concentration of seawater in clay conglomerates in the sedimentary basin, because there is no carbonate rock and sulfate rock of corresponding scale after the deposition of clay conglomerates in the basin; (3) clay conglomerates of MYJ Fm. were deposited in continental shallow water basin; (4) the matter source of potash minerals is deep marine strata; (5) in the MYJ Fm. sedimentation period, deep source salt moved to the surface under the background of extensional structure, and the subsequent sedimentary clastic rock formed a protective layer of potash-bearing rock, thus completing the “deep source and shallow mineralization” metallogenic process.

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The Simao Basin is one of the most important Mesozoic salt basins in China, and the salt rocks generally contain mud-clast conglomerates. The characteristics of mud-clast conglomerates can provide effective information indicating the evolution process of the salt deposits. A combined analysis of trace elements and clay minerals was performed to characterize the genetic model of mud-clast conglomerates in the salt rocks. The results show that the sedimentary materials are felsic rocks from the Upper Crust. Ternary plot diagrams show that the tectonic settings are between the continental margin and the continental island arc. The clay minerals in the samples mainly contain illite and illite–smectite mixed layers. A dry and hot climate prevailed during the deposition of the Mengyejing Formation, and the warm-humid climate that also occurred is interspersed in some periods. Evaporation makes the water bodies shallower, and the concentrated brine starts to precipitate salt under the arid and hot climate conditions. Redox proxies indicate that most of the samples were deposited in relatively oxic conditions. The Sr/Ba ratios (average of 0.5) and paleosalinity (average of ∼35‰) during the depositional period indicate an increase in terrigenous freshwater input and carry of clasts into the evaporation basin. Furthermore, the freshwater supply enhances the hydrodynamics; as a result, the detritus that deposited in the early stage is broken into mud-clast conglomerates and co-deposited with the salt rocks. The results provide a geochemical basis for further study of mud-clast conglomerates in the Simao Basin and supply recommendations for the origin of salt deposits in similar basins around the world.

In order to distinguish the source and migration direction of natural gas by geochemical characteristics of butane, the components and carbon isotopes of natural gas from major hydrocarbonbearing basins in China were analyzed. The results showed that: (1) Oil-type gas has i-C 4 /n-C 4 <0.8, 13 C butane <-28‰, 13 C i-butane <-27‰, 13 C n-butane <-28.5‰, whereas coal-type gas has i-C 4 /n-C 4 >0.8, 13 C butane >-25.5‰, 13 C i-butane >-24‰, 13 C n-butane >-26‰. (2) When 13 C i-butane-13 C n-butane is greater than 0, the maturity of oil-type gas is generally more than 2.4% and that of coal-type gas is greater than 1.4%, whereas when the difference is less than 0, the maturity of oil-type gas is generally less than 1.1% and that of coal-type gas is less than 0.8%. (3) When natural gas migrates through dense cap rocks, the value of i-C 4 /n-C 4 increases, whereas when it migrates laterally along a reservoir, the value of i-C 4 /n-C 4 decreases. (4) Sapropelic transition zone gas with composition and carbon isotopic signatures similar to those of oil-type gas in the low thermal evolution stage is found to have a relatively high butane content. (5) The values of i-C 4 /n-C 4 and 13 C n-butane 13 C i-butane of gas which has suffered biological degradation are significantly higher than those obtained from thermogenic and bio-thermocatalytic transition zone gas. Thus, natural gas of different genetic types can be recognized through component analysis and carbon isotopic signatures of butane, the natural gas maturity can be estimated from the difference in carbon isotopic content between isobutane and n-butane, and the migration direction of natural gas can be determined from i-C 4 /n-C 4 ratios and transport conditions, which can also be used to thermogenic and bio-thermocatalytic transition zone gas.