Sedimentary And Geochemical Characterization Of The Carboniferous Shale In Eastern Of Qaidam Basin

Qaidam Basin, which adjoin Qilian mountain range to the northeast part, Kunlun mountain range to the south part and Altyn Fault Zone, is located in the north of the Qinghai-Tibet Plateau. The research region in this article is in the eastern part of the Qaidam Basin, which is between South Qilian and Eastern Kunlun in space. According to the present tectonic framework, the region can be divided into four parts, respectively Delingha Depression, Huobuxun Depression, Xitie Mount-Maoniu Mount uplift and Eastern Fault Depression. At present, the exploration and developing extent of the Carboniferous strata in Qaidam Basin are in a low level partly result from lacking borehole material which reach the carboniferous strata. Moreover, the seismic exploration extent was also in a low level and it is difficult to identify the destination sequence. With the increasing of the extent of the oil exploration in the Mesozoic strata in Qaidam Basin, the Paleozoic strata especially Carboniferous strata attract the researchers’ attention. Through previous research work on this region, the geological comprehension about the Carboniferous strata in this region developed intensively so that many fruitful research achievements were attained. The previous study about the Carboniferous strata mainly concentrated on the ordinary sedimentary and structural characterization and the elementary organic geochemical characterization by the methods of field investigation.As a type of unconventional resources, shale gas has gained a relative high production and it has become an very important portion of the whole resource production as well as the a pivotal alternative of the conventional resource in USA. At present, the shale gas research extent in our country is in a low level and there is nearly no industrial production. Based on the previous work, the research region deposited shale with a relative wide range in a shallow marine environment, which becomes the fine research basement for the shale gas.On the basis of the adequate comprehension of present material of the research region as well as the thorough field investigation, combining with the new drilled borehole material, under the guidance of the theory of sedimentary, oil geology and organic geochemistry, we did a series of work on the sedimentary and organic geochemical characterization of the relative shale layer to estimate the shale gas resource potential.Through the field outcrops and the interpretation of the core material, on the basis of the accurate face or sedimentary environment interpretation of single geological section, we built the vertical strata sequence of carboniferous in research region, and recognize that the Carboniferous strata has a wide distribution in the Qaidam Basin which mainly deposited carbonates in the sedimentary environment of shallow-shelf or the clast in the shallow marine. We also analyzed the distribution and thickness of the shale in the vertical sequence which convinced that the depositional environment of shale was mainly in lagoon and tidal flat. What’s more, we discussed the lateral distribution of shale layer.With the experimental methods of XRD, we analyzed the mineral composition of the shale in research area. Based on the results from the 21 examples from two borehole, the clay minerals content ranges from 24%-43%, of which the average amount is 34%; the types of clay mineral mainly are kaolinite and andreattite. The brittle mineral mainly consists of quartz and the content ranges from 33%-66%, with an average amount of 52%, which indicate the shale layer has a fine mineability.According to the theory of organic geochemical, we analyzed the examples both from outcrops and borehole with experimental methods including rock pyrolysis, quantitive analysis of the hydrocarbon component, the analysis of the maturity of organic matter, and the type classification of the kerogen. Through these experiments, the conclusion is that the content of TOC varies in eastern of Qaidam. The data of 21 examples from two bores indicate the average content of TOC is 1.4%, which can be defined as fine-source rock after remove 3 abnormal high value. We also took 10 examples to identify the type of organic matter, the result of which conclude that the type of kerogen is mainly Ⅱ, as for the maturity of organic matter, the data shows that the Ro ranges from 1.22%-1.55%, with an average of 1.33%, which shows that the revolution of organic matter was in the stage of generating wet-gas.