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To elaborate genesis of the Ordovician Ma55 Sub-member dolomites in the Ordos Basin, this study implemented a series of analyses, including petrological investigation and geochemical (constant and trace elements, and isotopes) characterization. Three main types of dolomites were identified, namely micritic dolomite, micritic silty dolomite and silty dolomite. Specifically, the micritic dolomite is featured by dull red luminescence, low degree of order, high Sr isotope value, positive linear MgO–CaO correlation, high contents of Na and Sr elements, low contents of Fe and Mn elements, obvious negative δ13C excursion and slightly positive δ18O value, all of which indicate a syngenetic–penesyngenetic closed, high salinity environment. In comparison, the micritic silty dolomite is characterized by no or extremely dull red luminescence, moderate-to-high degree of order, higher δ13C value, slightly lower 87Sr/86Sr ratio, and sharp decrease in contents of K, Al, Fe and Mn elements, all of which indicate this type of dolomite to be the product of seepage–reflux dolomitization. Moreover, difference in geochemical characteristics between micritic silty dolomites in the depression and those in the platform flat suggests the Mg-rich brine from the tidal flat to be the dolomitization fluid for the platform flat while the Mg-rich brine from the overlying Ma54 Sub-member to be the dolomitization fluid for the depression. Meanwhile, the silty dolomite is observed to have no or extremely dull red luminescence, high degree of order, heavy δ18O value, high Sr isotope, lower Na content, and higher Fe and Mn contents, which are attributed to burial dolomitization in the fresh water environment. The distribution of dolomite was demonstrated to be controlled by sedimentary microfacies, and the dolomite in the West Jingbian dolomitic flat area turns out to have the largest distribution area and thickness as well as the best reservoir capacity. Massive fractures and vugs were formed in the dolomitization process, which were the basis of high-quality reservoir formation, involving generation of a relatively small amount of intercrystalline micropores during the syngenetic–penesyngenetic dolomitization, intercrystalline pores after the penecontemporaneous dolomitization, and intercrystalline and dissolution pores during the burial dolomitization. Moreover, dissolution during hypergene and burial periods contributed to most of the reservoir space, while filling effect during the burial period significantly reduced porosity.

Ha6 block is located in the north of the Tarim Basin. In this study, the characteristics of the Ordovician crude oil in the Ha6 block were analyzed by the gas chromatography-mass spectrometry (GC-M...

In this paper, the Cambrian crude oil and source-rock geochemistry in the ZS well field of Tazhong area, Tarim Basin, was analyzed using the gas chromatography-mass spectrometry (GC-MC). Mo...

The RePu area is located in the Tarim Basin, China. Based on the exploration and development, this paper analyzed the characteristics of the Ordovician crude oil and the source rocks by gas chromat...

Previous studies on graptolites have mainly focused on stratigraphic division and sedimentary environment analysis. On the basis of previous studies of Ordovician and Silurian graptolites in the Middle–Upper Yangtze region, this paper makes full use of core analysis, drilling geological, experimental analysis, and logging data from wells and combines graptolite types in different sedimentary environments to identify 4 graptolite zones, WF1–WF4, and 9 graptolite zones, LM1–LM9, in the Silurian Longmaxi Formation in the Middle–Upper Yangtze region, among which WF2–LM4 present high total organic carbon (TOC), organic pore development, and shale gas enrichment and are favorable development zones. This study reveals that enrichment in graptolites has positive correlations with TOC, organic porosity, and total hydrocarbon content. The evolution of graptolites has contributed to the enrichment in organic matter and the formation of organic pores in shales and affects shale gas–bearing properties to a certain extent, which is an important indicator for shale gas reservoir evaluation.

The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Babio, Cu/Al, Ni/Al, Siexcess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic&ndash, euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic&ndash, euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression&ndash, regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation.

In order to effectively evaluate shale gas reservoirs with low porosity, extra-low permeability, and no natural productivity, dipole array acoustic logging, which can provide various types of information including P-wave slowness (DTC) and S-wave slowness (DTS), is widely used. As the dipole array acoustic logging tool has a larger investigation depth and is suitable for complex borehole environments, such as those with a high wellbore temperature, high drilling fluid column pressure, or irregular borehole wall, it has been mainly applied to the evaluation of lithology, gas potential, fractures, and stimulation potential in shale gas reservoirs. The findings from a case study of the Sichuan Basin in China reveal that the acoustic slowness, S-P wave slowness ratio (RMSC), and S-wave anisotropy of the dipole array acoustic logging can be used to qualitatively identify reservoir lithology, gas potential, and fractures. Using the relationship between DTC and the total porosity of shale gas reservoirs, and combined with the compensated neutron (CNL) and shale content (Vsh) of the reservoir, a mathematical model for accurately calculating the total porosity of the shale gas reservoir can be established. By using the relationship between the RMSC and gas saturation in shale gas reservoirs and tied with density log (DEN), a mathematical model of gas saturation can be established, and the determination of gas saturation by the non-resistivity method can be achieved, delivering a solution to the issue that the electric model is not applicable under low resistivity conditions. The DTS, DTC, and DEN of shale can be used to calculate rock mechanic parameters such as the Poisson’s ratio (POIS) and Young’s modulus (YMOD), which can be used to evaluate the shale stimulation potential.