Sequence Stratigraphy, Sedimentary Characteristics of Barrier Coastal Sedimentary System of the Benxi Formation (Gaoqiao Area, Ordos Basin) and Implications for Gas Exploration

Benxi Formation is an important abundant natural gas resource in Gaiqiao area. Therefore, to analyze the sequence and sedimentary characteristics of the Benxi Formation and to provide guidance for predicting the favorable gas reservoir distribution. In addition, after comprehensive use of core, thin section, drilling, and logging data, the sedimentary microfacies of Benxi formation were studied. The Benxi Formation in Gaoqiao area is concluded to be a barrier coastal sedimentary system, subdivided into eight sedimentary microfacies, namely, tidal delta, tidal channel, sand flat, mixed flat, mud flat, peat flat, barrier bar, and lagoon mud microfacies. Along with sequence framework and sedimentary microfacies characteristics, planar distribution characteristics of sedimentary microfacies and vertical sequence characteristics are also studied. It is concluded that i) From west to east, peat flat - mud flat - mixed flat - sand flat - lagoon mud - barrier bar have been developed in the study area ii) Four typical vertical sequences have been identified: MA1, MA2, MA1, MA4. The first three sequences have developed extensively and completely in the study area, while the last one generally has one or more microfacies missing; iii) The sedimentary system of the Benxi Formation in Gaoqiao area is mainly vulnerable to tectonism, sequence, and sea-level changes. The Second Member is the target gas-producing strata of the formation. Both barrier bar and sand flat microfacies have a good gas display. However, the barrier bar microfacies are the most favorable natural gas distribution zones with giant and premium gas reserves.