Upper Carboniferous unconformity and reservoir development model of the western Maigaiti area, Tarim Basin

In the western Maigaiti area in the Tarim Basin, the Carboniferous oil and gas reservoirs discovered in the Bashituo and Yasongdi areas showed a good prospect for hydrocarbon exploration. However, their development models are not clear. Unconformity characteristics were analyzed through seismic and drilling/logging stratigraphic comparison, and reservoir development features were studied combing core and seismic data. The western Maigaiti area uplifted and was eroded during the Bachu Movement in the Late Carboniferous, forming two unconformities including T₅⁵ (between the Kalashayi and Xiaohaizi formations) and T₄⁵ (between the Xiaohaizi and Nanzha formations). The T₅⁵ unconformity is a tectonic unconformity, and mainly formed karst geomorphologies such as uplift, slope and valley. It developed two karst systems, the vadose zone and shallow undercurrent belt, and the deep undercurrent belt. Favorable reservoirs mainly locate on karst slopes. The T₄⁵ unconformity is a stratum unconformity controlled by paleotopography and eustatic movement. Dolomitic reservoirs and fresh water leaching reservoirs developed in intraplatform shoal with high landform in the highstand system tract in the Xiaohaizi Formation.