Sedimentary architecture and distribution of intra-platform shoal in sequence framework of Permian Changxing Formation in central Sichuan Basin,SW China

Based on the comprehensive analysis of core, thin section, logging and seismic data, this study carried out the identification and comparison of Permian Changxing Formation sequences, clarified the typical sedimentary architectures of intra-platform shoal, investigated the vertical and horizontal development and distribution of intra-platform shoal in each sequence, and thus established the sedimentary evolution model of shoal body. The study results are reflected in four aspects.First, there are two complete third-order sequences(SQ1 and SQ2) in Changxing Formation in central Sichuan Basin. SQ1 is generally thick in the north and thin in the south, and SQ2 shows a thickness differentiation trend of “two thicknesses and three thinnesses”. Second, the Changxing Formation in central Sichuan Basin mainly develops intra-platform shoal, inter-shoal sea and intra-platform depression subfacies. In the vertical direction, the intra-platform shoal mainly presents two typical sedimentary sequences: stable superposed and high-frequency interbedded. Third, the stable superimposed sedimentary sequence is developed in the shoal belt at the edge of intra-platform depression, which is composed of two shoal-forming periods and located in the highstand systems tracts(HSTs) of SQ1 and SQ2. The high-frequency interbedded sedimentary sequence is developed in the southern shoal belt of intra-platform depression, which is composed of four shoal-forming periods and mainly located in the HST of SQ2. Fourth, during the SQ1 deposition, the intra-platform shoal was mainly developed at the edge of the intra-platform depression on the north side of the study area, and the inter-shoal sea subfacies was mainly developed on the south side. During the SQ2 deposition, the intra-platform shoal was widely developed in the area, forming two nearly parallel intra-platform shoal belts. The study results provide direction and ideas for exploration of Changxing Formation intra-platform shoal reservoirs in central Sichuan Basin.

Pressure transient analysis of multiple fractured horizontal wells in naturally fractured unconventional reservoirs based on fractal theory and fractional calculus

Currently,most models for multiple fractured horizontal wells(MFHWs)in naturally fractured unconventional reservoirs(NFURs)are based on classical Euclidean models which implicitly assume a uniform distribution of natural fractures and that all fractures are homogeneous.While fractal theory provides a powerful method to describe the disorder,heterogeneity,uncertainty and complexity of the NFURs.In this paper,a fractally fractional diffusion model(FFDM)for MFHWs in NFURs is established based on fractal theory and fractional calculus.Particularly,fractal theory is used to describe the heterogeneous,complex fracture network,with consideration of anomalous behavior of diffusion process in NFURs by employing fractional calculus.The Laplace transformation,line source function,dispersion method,and superposition principle are used to solve this new model.The pressure responses in the real time domain are obtained with Stehfest numerical inversion algorithms.The type curves of MFHW with three different outer boundaries are plotted.Sensitivity analysis of some related parameters are discussed as well.This new model provides the relatively more accurate and appropriate evaluation results for pressure transient analysis for MFHWs in NFURs,which could be applied to accurately interpret the real pressure data of an MFHW in field.