--> Abstract: Outcrop Cyclostratigraphy As Analogue For The Mississippian Madden Deep Gas Reservoir, by H. Westphal, G. P. Eberli, and G. M. Grammer; #90928 (1999).

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WESTPHAL, HILDEGARD1, GREGOR P. EBERLI1, and G. MICHAEL GRAMMER2
1University of Miami, RSMAS, Miami, FL
2Texaco EPTD, Houston, TX.

Abstract: Outcrop Cyclostratigraphy as Analogue for the Mississippian Madden Deep Gas Reservoir

Surface exposures of the Mississippian Madison Formation about 20 km to the north of the Madden Field in the Wind River Basin (Wyoming) provide excellent outcrop analogues to the deeply buried gas field (>24,000 ft.). In outcrop, the dolomitized portion of the Madison Formation displayed good reservoir quality along the entire regional cross-section of approximately 100 km, but lateral and vertical facies variations within individual sequences and depositional cycles produce heterogeneities on the reservoir scale.

Spacing of four outcrop study sites along an east-west trending transect was chosen in a way to assess large-scale variations (20-40 km) as well as small-scale reservoir heterogeneities. Three of these locations are aligned along strike on the paleo-shelf, while one outcrop site is slightly downdip, exhibiting a more open marine facies and greater thickness. In the study area the 120 m thick strata of the Madison Formation is organized into four sequences, with two basal transgressive sequences overlain by two regressive sequences. The sequences appear laterally homogeneous on outcrop scale with only slight compositional variations where biological buildups are involved. The two transgressive sequences are intensively dolomitized and comprise the main reservoir unit in the subsurface.

Each of these sequences is composed of meter-scale cycles that are the fundamental unit to determine the Madison Formation reservoir characteristics. A transgressive and regressive portion of deposition can be recognized in each cycle. The transgressive hemicycles are dominated laminated tidal fiat mud-wackestone to stromatolites and subtidal deposits, while the regressive portion is characterized by high-energy carbonate sand shoal facies. The proportion of transgressive to regressive facies varies within each cycle according to the longer-term trend. These lithologic variations in conjunction with the variations in the dolomitization are responsible for the heterogeneities in each cycle, i.e. on a reservoir scale. Core and log-data of the deep gas reservoir correlate well with the depositional sequences found in the outcrop. Consequently, we feel confident that the fine-scale architecture of the outcrops can be used to assess the heterogeneities within the reservoir.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas