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Bitumen Anomaly Mapping (BAM) With an Integrated Basin Analysis Sequence Stratigraphic Approach for Delineating Hydrocarbon Migration Sweet Spots in Shales: Eagle Ford, South Texas Example


Basin analysis of high resolution sequence stratigraphy is applied to 27 boreholes penetrating the Lower Cretaceous Eagle Ford Formation of South Texas. With measured elevated total organic carbon (up to 10.9 wt.% TOC), a Type II kerogen organofacies, and a range of thermal maturities but with significant horizontal and vertical variations, determining the regional basin and sequence stratigraphic setting of hydrocarbon deposition, maturation, and migration better allows the prediction of organic sweet spots ahead of the drill bit. Local variations in autocyclic processes are minor as the Lower and Upper units of the Eagle Ford can be divided into five distinct parasequence sets based upon gamma ray trends which correspond one to one with allocyclic fluctuations in the global eustatic sea level curve. However, determining depth and horizontally thermally dependent liquid gas boundaries through basin modeling kinetics does not correspond one to one with geochemical maturity analysis (pyrolysis data) nor to paradigms of transgressive system tract (TST) dominating organic richness. Indeed, of the five parasequence sets, the regressive systems tract 1 (RST 1) and transgressive systems tract 2 (TST 2) exhibit the highest measured total organic carbon (TOC) and bitumen (S1) content. Incorporating bitumen anomaly mapping (BAM), the integrated basin analysis, sequence stratigraphic, and geochemical data point to significant bitumen migration. The BAM procedure is an empirical method of calculating S1 anomalous values by suppressing maturity, TOC, and organofacies biases. Application of BAM to the Eagle Ford directly delineates depleted intra-formational source pod and enriched reservoir accumulations. Maps of the parasequence set dependent bitumen and bitumen anomalies are indicative of a variety of geologically static and dynamic organic “sweet spots.” Using structural information from 3D seismic of the parasequence sets, transgressive systems tract 4 (TST 4) provides the highest observed directly measured bitumen anomalies in the region and directly correlates to basin modeled bitumen migration source pods, pathways, and traps. In particular, while the value of the application of integrated basin modeling, high resolution sequence stratigraphy, and the generation of BAM to the Eagle Ford is demonstrated, in general the integrated application may have potential for the designation of migration in unconventional shales elsewhere.