--> --> Core vs. cuttings – building understanding from an underutilized dataset to more fully characterize producing reservoirs within the Cretaceous Almond Formation, Southwest Wyoming

AAPG Rocky Mountain Section Meeting

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Core vs. cuttings – building understanding from an underutilized dataset to more fully characterize producing reservoirs within the Cretaceous Almond Formation, Southwest Wyoming

Abstract

Tight reservoirs with complex diagenetic histories are becoming increasingly important for oil and gas exploration and production as conventional, high porosity, and high permeability reservoirs deplete. Understanding the fine-scale heterogeneity of tight reservoirs is critical for predicting flow rates, recovery efficiency, and targeting sweet spots. Whole core is an effective means of acquiring ground-truth datasets but is limited by cost and geographic distribution. Drill cuttings, in contrast, are inexpensive and collected ubiquitously. They represent an under-utilized dataset for assessing reservoir quality, calibrating open-hole log data, and reducing exploration risk. 164 thin sections of Cretaceous Almond Formation strata were made from 16 wells within the Green River Basin of Southwestern Wyoming. 85 of these thin sections were made from whole core and 79 were made from cuttings. Thin sections from two calibration wells were prepared with both cuttings and core over the same interval. Thin sections were analyzed for grain, cement, and pore type, as well as average pore and grain size. Comparisons between thin sections of cuttings and core from the same interval demonstrate that Almond Formation cuttings in thin section are identifiable based on primary and diagenetic mineralogy. The drilling process biases the cuttings toward an increase in quartz (47% to 57%), a decrease in clay cement (8% to 1%), and a decrease in primary and secondary porosity (8% versus 4%). Bias is also recorded according to bit type. Newer PDC type bits preferentially disaggregate or destroy minerals less resistant than quartz. Tri-cone drill bits better preserve primary and secondary porosity. This methodology of utilizing cuttings allows for geographically expanded reservoir characterization and prediction in areas where minimal core is available. Once the biases in the cuttings are measured and quantified, they can be used to bolster geologic interpretations in exploration and development applications.