Study On Sequence Stratigraphy, Sedimentary Fades And Reservoir In Shanxi Formation In Daniudi Das Field, Ordos Basin

The Early Permian Shanxi Formation is one of the most important units with high gasproduction in the Dniudi Gas Field, which is a typical of tight sandstone reservoirwith high heterogeneity in Ordos Basin. Based on investigation of cores, outcropsediments, well logs, seismic data, analysis of major elements, trace elements andoxygen isotope in the samples of mudstone, and combined with paleontology, thinsections, scanning electron microscopy, we carried out the study on the sequencestratigraphy, sedimentary facies and reservoir evaluation in the Shanxi Formation.Different from the division schemes of the sequence stratigraphy proposed byprevious researchers, the Shanxi Formation was divided into four third-ordersequences from base to top as SQ1, SQ2, SQ3and SQ4. The sequence boundaries(SQ) were identified by the planes of unconformity at the outcrop and responses ofthe cores and well logs in the Daniudi Gas Field. We proposed that sequenceboundaries are the erosion surfaces between the transitional sediments with theunderlying coalbed in the Shanxi Formation. Based on investigation of sedimentarycharacteristics changed at the outcrop, responses of cores and well logs, analysis ofgeochemistry of element and stable isotope geochemistry, we proposed that the firstflooding surfaces (FFS) of the four sequences are the bottom boundaries of the muddysediments which deposited during the process of the relative sea-level rise. As well asthe top surface of the muddy sediments were identified as the maximum floodingsurfaces (MFS). The highstand systems tracts (HST) deposits are fine-grainedsediments with an aggradational parasequence or coalbed in the Daniudi Gas Field.We carried out the study on sedimentary facies with different lithologies asconglomerate, mudstone and coalbed. We identified three types of sedimentary faciesin the Shanxi Formation, such as transitional facies-braided delta front, marinefacies-offshore and swamp facies. We also identified four microfacies includingsubaqueous braided channel, subaqueous interdistributary, mouth bar and sheet bar inthe braided delta front. Comparing with the sedimentary characteristics of theHeidaigou outcrop located in the north of Ordos Basin, we proposed that there were four times of braided delta front sediments which formed a progradation in thelowstand systems trcts (LST) in each sequence in the Daniudi Gas Field. The muddysediments formed the transgressive systems tracts (TST) in each sequence. Thehighstand systems tracts deposits are swamps in the SQ1, SQ2and SQ3and braideddelta front in SQ4.We proposed that the geological factors which controlled the property of thereservoir in Shanxi Formation include compaction, carbonate cementation and theprogradation of the braided delta. On the other side, the organic acids from the thickcoal made the reservoir with high prosity as well as high gas production. On the basisof research of the controlling factors of reservoir properties, we proposed a newgrading standard of the Shanxi Formation reservoir, and divided it into threecategories with eight levels.