Abstract

Lancelot, Guinevere, and Excalibur are Rotliegende Group gas fields that were developed mainly by horizontal drilling. Horizontal well geological data from these fields are interpreted and integrated with vertical well data to evaluate the target zone and enhance the reservoir description. Lateral variations in reservoir quality and eolian facies are interpreted in a horizontal core. Borehole images and conventional electric logs are used to identify facies, record the lateral extent and orientation of eolian dune cross-bed sets, characterize diagenetic trends, and locate fractures through the uncored horizontal well sections. The cored interval consists of gently dipping, medium-grained dune base strata separated by a set boundary from moderately steeply dipping, fine-grained dune top sediments. Permeability varies with grain size and is therefore greater in the dune base than the dune top in this cored section. The main facies in the horizontal wells as a whole are of eolian dune, interdune, and fluvial to marine reworked eolian origin. Comparison of facies in horizontal and vertical wells defines reservoir units in the upper part of the formation that show large-scale mound and intermound infilling geometries, reflecting relic dune field geomorphology. Comparison of the lateral extents and heights of eolian cross-bed sets and patterns of foreset bed dip vector swing suggest that the sets are strongly trough shaped in geometry. Locally, very large sets of eolian cross-bedding are preferentially cemented. In the uppermost reservoir unit of fluvial to marine reworked sandstones, cementation increases in intensity upward parallel to top Rotliegende, a characteristic seen clearly due to the gradual climb of horizontal well sections through the reservoir. Cemented and apparently open fractures occur in these wells. The former are natural fractures, whereas the latter are a drilling-induced phenomenon.

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