Three-dimensional fades architecture and three-dimensional calcite concretion distributions in a tide-influenced delta front,Wall Creek Member,Frontier Formation,Wyoming

摘要

Ground-penetrating radar (GPR) has been used to image the three-dimensional (3-D] internal structure (and,thus,the 3-D facies architecture) of a top-truncated delta front in the topmost parasequence in the Wall Creek Sandstone Member of the Frontier Formation in Wyoming and to estimate the distribution of low-permeability concretions throughout the 3-D GPR volume.The interpretation of the 3-D GPR data is based both on correlations with outcrop and on calibration with core data from holes within the survey grid.Two main radar facies (RF) are identified.Radar facies 1 corresponds to tide-influenced mouth bars formed by a unidirectional flow during delta progradation or bidirectional flow during tides,whereas RF2 is correlated with laterally migrating channels developed on previous bar deposits.The delta-front foreset beds dip in the same direction as the dominant paleocurrent indicators.The GPR interpretation is consistent with the outcrop interpretation that,following a regressive period,bars and channels were developed at the Raptor Ridge site before subsequent trans-gressive ravinement.The individual 3-D deltaic facies architectures were reconstructed from the 3-D GPR volume and indicate that the depositional units are larger than the survey grid. Cluster analysis of the GPR attributes (instantaneous amplitudes and wave numbers) calibrated with the cores and the outcrop was used to predict the distribution of near-zero permeability concretions throughout the 3-D GPR volume;clusters of predictive attributes were defined and applied separately in the bars and channels.The predicted concretions in the bars and the channels are 14.7 and 10.2% by volume,respectively,which is consistent with those observed in the cores (14.7 and 10.5%,respectively),and their shape and thickness are also generally in consonance with those in the outcrop and cores.The estimated concretions are distributed in an aggregate pattern with irregularly shaped branches within the 3-D GPR volume,indicating that the cementation does not follow a traditional center-to-margin pattern.The concretions and 3-D geological solid model provide cemented flow baffles and a 3-D structural framework for 3-D reservoir modeling,respectively.