LAUSR

Abstract For sedimentary analysis, core observation is the most direct and efficient method, but its high cost and low coverage call for a less expensive and more abundant alternative. Fortunately, FMI (Formation MicroImager) and conventional logs cover this shortage due to the continuity and high resolution of FMI images combined with the low cost and sufficient data of conventional logs. Here, a feasible method for sedimentary microfacies analysis via the combination of FMI logs and conventional logs is provided in the Middle Ordovician Tahe Oilfield in the Tarim Basin, China. Five microfacies assemblages have been recognized based on analysis of the FMI logs from 7 wells calibrated by cores: patch reef (MA1), shoal (MA2-4), and intershoal (MA5) assemblages. An FMI geological interpretation model of microfacies assemblages has been established. Based on the analysis of conventional logs combined with the FMI geological interpretation model from 5 wells, the deep investigation dual lateral resistivity (RD) log, gamma ray (GR) log, and density (DEN) log are all sensitive to sedimentary microfacies, and among them, the RD log is considered the most sensitive conventional log. Four logging facies are recognized, including reef and high-energy shoal complex logging facies (LF1), reef and middle-energy shoal complex logging facies (LF2), reef and low-energy shoal complex logging facies (LF3), and intershoal logging facies (LF4). The results show that the combination of FMI logs and conventional logs is a feasible method for sedimentary microfacies analysis. It improves the recognition accuracy of sedimentary microfacies, and it expands the use of FMI logs in the area of carbonate microfacies interpretation. To some extent, it improves knowledge of the distribution of sedimentary facies in the Ordovician strata of the Tahe Oilfield, Tarim Basin, China.