The Depositional Model of Sandy Beach Bar on the Far Bank of the Inland Depression Lake Basin

Key words: 
• Tainan area /
• beach-bar /
• grain size /
• sedimentary micro /
• formation mechanism /
• depositional model

Abstract: [Objective]Typical beach bar sedimentary sand bodies are developed in the middle and upper member of Shibei Formation in Tainan area of Qinghai Oilfield, which is an important high-quality reservoir. The formation mechanism and distribution pattern of beach bar sand body have always been controversial, which has limited the further exploration and development of the gas field. Therefore, it is urgent to carry out fine characterization of the interior of the beach bar in Tainan area, identify the sedimentary microfacies types, and establish the corresponding deposit model of the beach bar. [Methods] Under the guidance of sedimentology, considering the hydrodynamic difference during the formation of beach bars, the sand body particle size and permeability are different. Using the median grain size as the link, the corresponding logging template of rock facies is summarized, and the interpretation standard and dynamic response of sedimentary microfacies are determined, so as to classify the sedimentary microfacies more scientifically. Based on the modern sedimentary analogy, sand body anatomy in the dense well pattern area of the study area, and the reasonableness of microfacies classification by horizontal Wells and production data, a beach bar sedimentation model suitable for the inland depression lake basin is proposed considering that the formation of beach bars is controlled by multiple factors. [Results] The beach and bar subfacies in the middle and upper part of Sebei Formation in Tainan area can be further divided into the main body of the bar, the edge of the bar, the main body of the beach and the edge of the beach. The bar sand is distributed above the beach sand, extending vertically to the northwest wind, in the form of lenticular and mencrescent, with a length of about 1,250 m and a width of about 250 m. The linear positive correlation between the length and width of the bar sand is about 5, which is consistent with modern sedimentation. As a whole, the beach sand is distributed in the direction of vertical provenance, and broadly and gently covers the edge of the bar sand. By comparing the deposit of beach bar in Tiangiz Lake, the extension direction, distribution rule and length-width ratio of bar sand are well corresponding. Sufficient source supply provides a good material basis for the formation of beach bars. The coupling of palaeostructure, palaeogeomorphology, palaeomonsoon and palaeowater depth controls the development scale of beach bars, the development form and geographical location of bar sands. [Conclusions] The sedimentary model of "open shallow lake, wind wave transformation, oblique row type" is proposed, which can provide theoretical guidance for the identification of beach bar system in inland depression lake basins.