ABSTRACT: Integrated Geologic and Geophysical Mapping: An Example from the Muddy Sandstone Wind River Basin, Wyoming

Samuel R. Johnson, Steve K. Western

Muddy Sandstone stratigraphic traps have been the primary stratigraphic exploration target in the Wind River basin (WRB) for BP Exploration and many in industry since the discovery of the Grieve field (30 MMBO, 60 bcf of gas) in 1954. Since 1984 in the eastern WRB, 18 wildcats have been drilled resulting in three new field discoveries which total approximately 23 MMBOE. BP Exploration staff has successfully predicted the reservoir outcome of over 95% of the wildcats by incorporating log facies and facies models with seismic amplitude mapping of the Muddy reflector.

The Early Cretaceous Muddy Sandstone unconformably overlies the earlier Cretaceous marine Thermopolis Shale and is conformably overlain by the marine Mowry Shale. An incised valley system was cut into the Thermopolis Shale during a eustatic sea level drop. During the ensuing sea level rise in the southeast portion of the WRB, the Recluse sandstone is the primary Muddy reservoir unit. Secondary Muddy sandstone deposition of marine shoreline sandstones occurred during fluctuations of the primary eustatic sea level cycle. Therefore, in certain areas of the WRB, the Recluse member sandstones can be overlain and/or underlain by marine sandstones.

In areas where the older and younger marine sandstones are absent, seismic amplitude mapping, incorporating well control, provides a method of mapping the extent of the valley fill sediments. The Muddy valley-fill sediments are generally less than 30 m thick and are encased in slower velocity shales. When valley-fill sediments are present, the Muddy reflector is high amplitude in contrast to low amplitudes when the valley-fill is absent. Estimates of valley fill lithology are provided through synthetic modeling of well logs representing different lithofacies. The resulting wavelet characterization, which includes wavelet shape and peak-to-trough time separation, improves predictability of sandstone distribution within the Muddy valley system.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990