A Novel Approach to the Classification of Deepwater Mudrocks: Application of a New Paleoenvironmental Model Based on Biofacies

Keith A. Knabe, R.T. Beaubouef, Y. Chen, and T.C. Huang
ExxonMobil Exploration Co, Houston, TX

ExxonMobil Best Practices for evaluating environment of deposition (EOD) parameters in deep-water hydrocarbon systems, in the past, focused on the classification and characterization of coarse-grained reservoir facies. Non-reservoir sediments (shales, mudstones) did not receive as much focus despite their importance as source and seal components within hydrocarbon systems. Recent work from exploration wells, however, suggests that the reservoir and non-reservoir facies-types in the deep-water are genetically linked and that they should be studied as a whole using an integrated methodology.

Paleontologic analyses of deep-water sediments are providing new insights into how mudrocks are deposited. Based on the distribution patterns of selected microfossil groups, deep-water facies can be categorized according to their biofacies "fingerprint". In turn, these biofacies types can be related to geologic processes characteristic of different depositional provinces. Therefore, we are establishing important links between; 1) paleoecology, 2) sedimentary processes, and 3) EOD. Results of biofacies analysis have prompted us to reconsider concepts concerning the occurrence, distribution and properties of deep-water shales and mudstones and their relationship to well known reservoir units.

We propose that integration of paleontologic biofacies with analysis of seismic facies, cross-sectional geometry, map-patterns and log data enhances the reliability of EOD interpretations. EOD confidence impacts the risk related to the presence and quality of source, seal and reservoir.