Innovative Fluid Characterization Methodology for Ultradeep Tight Reservoirs in Different Formation Types: Tarim Basin Case Studies
Authors: Huan Zhipeng , Zhang Wei , Ni Xiaowei , Ai Yong , Cai Liang , Wang Wei , Wu Jin Long , Cao Yingchang 
Published: February 2025

Abstract
The Tarim Basin, located in the Xinjiang Uygur Autonomous Region of China, is renowned for its substantial oil and gas reserves. The geological formations in the basin span various geological periods, contributing to its rich hydrocarbon resources. The reservoir is located deep to more than 8000 meters vertically and consists of various lithologies. The reservoirs are rich in fractures but have tight petrophysical properties. Fluid identification is crucial to locate the sweet spot for exploration and development. Resistivity is the most common and straight-forward method for fluid identification, however, because of the mixed effects from pore structure, the influence of the surrounding rocks formation, sedimentary dips, and far-end fractures etc., hydrocarbons and water cannot be easily distinguished based on the resistivity difference. In tight carbonates, the Archie equation is not a satisfactory solution due to the reservoir's strong heterogeneity in mineralogy and pore structure. The compressional coefficient and Poisson's ratio do not exhibit an obvious crossover effect because of the compaction effect on rock sonic waves. New wireline logging techniques were developed to provided non-electrical solutions, including T1-T2 based nuclear magnetic resonance (NMR), advanced spectroscopy and pulsed neutron measurements. T1-T2 based 2D NMR measurements provide porosity and permeability information from T2-based analysis. When used in conjunction with T1 -based measurements, fluid identification through different T1-T2 response offers an advantage for distinguishing hydrocarbons and water. The continuous measurement enables separation and quantitation of different fluids within the pore system throughout the entire interval of the targeted reservoir. Advanced spectroscopy uses both capture and inelastic gamma ray spectroscopy measurements, providing precise dry weights for elements and minerals, along with total organic carbon (TOC). It also provides thermal neutron cross section (sigma) and chlorine concentration measurements (DWCL), which are suitable for fluid characterization while the formation water is saline. A new measurement called fast neutron cross section (FNXS) was introduced based on inelastic gamma ray measurements. FNXS is not sensitive to liquid- filled porosity variations but is sensitive to gas- filled porosity variation, which can act as a gas indicator. Case studies presented demonstrated how a novel methodology based on non-electric open hole logs reveals the fluid type in tight reservoirs in this ultradeep environment, the interpretation results matched well with drill stem tests, providing a novel solution to finalize the sweet spot interval for the targeted reservoir. The results show the compatibility of this workflow across different formation types and reservoirs. This paper presents a successful and novel integrated workflow that combines multiple wireline measurements for fluid identification in tight reservoirs, thereby lowering the uncertainty for water saturation estimation. This result has helped to enhance the methodology for fluid identification and has generated considerable economic value by escaping the water zones in the further development of the reservoir.