Microbial geochemistry of a complex piedmont zone in the southern Keping area, Tarim Basin

In complex geological settings, seismic data often suffer from limited imaging accuracy and a strong multiplicity of possible interpretations, making it difficult to conduct effective hydrocarbon detection and fluid identification. By investigating the southern Keping area, Tarim Basin, we assess the applicability of the microbial oil survey technique (MOST) to complex piedmont zones, aiming to develop a novel, integrated multi-technique approach to hydrocarbon exploration. By analyzing the content and distribution of surface microorganisms, we predict the subsurface hydrocarbon enrichment zones and identify the fluid properties. A comprehensive assessment is conducted by combining microbial data with seismic data. The results indicate the presence of five stable microbial anomaly zones within the study area. The comprehensive assessment based on microbial and geochemical indicators reveals that the study area can be divided into three categories of play fairway, i.e., classes Ⅰ, Ⅱ, and Ⅲ anomaly zones. Class Ⅰ anomaly zones, among others, are characterized by high microbial values (MVs), optimal hydrocarbon preservation conditions, and favorable structural trap conditions. Class Ⅱ anomaly zones exhibit high MVs and favorable preservation conditions but moderate structural trap conditions. In contrast, microbial anomaly zones with poor structural trap conditions are classified as class Ⅲ. The distribution of the anomaly zones has been confirmed by subsequent drilling, exhibiting a high degree of consistency. The microbial geochemical technique has yielded encouraging application results in the complex piedmont zone. The MOST, which directly reflects the present-day hydrocarbon leakage activity based on the abundance of hydrocarbon-oxidizing bacteria (HOB), is highly sensitive to dynamic hydrocarbon preservation conditions, making it suitable for the rapid delineation of play fairways. In contrast, the geochemical technique, i.e., sorbed soil gas (SSG), captures long-term hydrocarbon leakage by detecting historically adsorbed hydrocarbons. MOST and SSG thus complement each other, forming a dual-indicator model that considers both dynamic charging and historical preservation. This model allows for the assessment and classification of play fairways in cases where no reliable direct hydrocarbon indicators (DHIs) are available. Overall, a novel seismic-microbial-geochemical integrated model has been established for the southern Keping area, in which the tectonic framework is defined using seismic data, optimal exploration targets are primarily selected using MOST, and the evolution of hydrocarbon preservation conditions is validated using SSG. This integrated model can provide new ideas for hydrocarbon exploration in the complex piedmont zone.