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Understanding the Impact of Depositional Processes and Environments on Reservoir Quality in Deepwater Reservoirs: A Case History from the U.S. Gulf of Mexico

Abstract

Active exploration and development of deepwater turbidite reservoirs is taking place in many parts of the world. Billions of barrels of oil have been discovered but producing resources from these geologically complex deposits is challenging. The variety of rock types and stacking patterns gives rise to reservoir characterization uncertainties, and prediction of the extent of high and low permeability reservoir units is not straightforward. In the Gulf of Mexico we have established an important link between primary depositional characteristics and reservoir quality which we believe is widely applicable to other deepwater turbidite deposits around the globe. Detailed core description work of over 3,700 ft. of core from multiple reservoirs was carried out. In addition to our deepwater cores, we acquired data from multiple outcrop and modern analogue studies. Integrated core and outcrop studies established lithotype stacking patterns which are diagnostic for the larger scale genetically related elements (architectural bodies, referred to here as Genetic Elements) in the depositional system. By integrating reservoir quality data in the genetic element scheme, we were able to distinguish specific rock property suites and textural characteristics for the trunk channel, channelized lobe and distal lobe margin & fringe environments. Porosity and permeability data reveal that trunk distributary channels have some of the best reservoir quality. The channelized lobes mostly yield good permeability although its range straddles a few orders of magnitude for a relatively small porosity range. Permeability is overall lower in the lobe margins and fringes, however, porosities are similar to the channelized lobes. Textural characteristics specific to each genetic element are identified as the main cause for the observed reservoir quality differences. Samples from trunk channels are generally coarser-grained and have a lower abundance of silt-sized particles (average 24%) dispersed in the sand. Silt content systematically increases down-dip with an average of 32% silt in the channelized lobe environment and overall very fine-grained and abundant silt (average 47%) in the distal lobe margins and fringes of the fans. Clay content in these particular gravity flows is mostly low. Only in the distal margins and fringes there is somewhat more clay. As a best practice, reservoir quality in turbidites should be examined in submarine fan's architectural element context.