High Resolution Sequence Stratigraphy Analysis in a Carbonate Reservoir from Borehole Resistivity Image: a Case Study from Tarim Basin, West China
Authors: Yang S. , Xiao C. , Wei G. , Le Nir I. 
Published: May 2016

Abstract
Formation heterogeneity due to fractures, vugs, and mixed lithologies complicates the characterization of carbonate reservoirs. The lithology distribution is controlled by multiple factors, such as sediment source, depositional environment, and diagenesis. In addition, fracture development is influenced by lithology, burial depth, local structure, and far-field stress.
High-resolution sequence stratigraphy is one of the advanced methods that can be used to solve the lithology challenge. The method combines core analysis, conventional logs, outcrop studies, and seismic data. However, the analysis results are frequently constrained by the low resolution of the seismic and conventional log data and by limited core data.
A new workflow for high-resolution sequence stratigraphy analysis integrates borehole resistivity images with seismic, log, and core data. First, the borehole resistivity images are compared with core data, and the depositional facies are identified from calibrated resistivity image data combined with multiple-domain data. Second, sequence stratigraphic surfaces are identified from seismic and image data and the thicknesses of crossbedding and sequence cycles are used to classify the strata stacking patterns. Finally, the distribution of depositional environments within a sequence stratigraphy framework is analyzed by integrating the sequence stratigraphy patterns with seismic attribute maps and petrophysical log interpretation to predict the sweet spot.
This new approach was implemented in Block A8 of the Tazhong uplift in the Tarim basin. Six different depositional facies were identified from the core data from three wells and applied to an additional four wells and to noncored intervals. Isopach maps of the first long term sequence cycle were used to estimate the size of buildups (reef, mound) and predict the vug distribution. A recently drilled well confirmed the analysis results. This workflow can be applied to similar thick carbonate reservoirs in the shoal-reef margin of a carbonate platform.

Introduction
The Tarim basin has three main belts: the Mesozoic and Cenozoic foreland, the Tabei uplift, and the Tazhong uplift (Xianming Xiao et al. 2004). The Tazhong uplift, which has an area of 30 000 km2 is the most complex and difficult area for exploration. The Tazhong-1 slope-break zone extends 200 km from east to west adjacent to a major thrust fault zone in Lower to Upper Ordovician carbonate rocks. Much of the oil and gas discovered in the area occurs in the Upper Ordovician Lianglitage formation.
A complex shelf depositional system developed during deposition of the Lianglitage formation. At the same time, the continental shelf margin subfacies along the Tazhong I faulted slope-break zone was developed, and high-energy reef, shoal, and mound facies developed. Within the Tazhong shelf, tidal flat, gently sloping shelf shoal, shelf wash, and mound subfacies developed.
The study area, block A8, is located in the western part of the Tazhong I slope break. The first well was drilled in 2005. Eight wells have been drilled, and three of these show good production (Fig. 1). One more appraisal well was recently drilled.