Region-Based Basin Modeling: Correlating Depositional Environment to Connected Pore Volume, Powder River Basin (Prb), Wyoming and Montana

Jesse Melick and Michael Gardner
Earth Sciences, Montana State University, Bozeman, MT

Accurate geologic-based rock property distribution remains elusive, particularly for basin-scale models. Current methods to determine connected pore volume in a sedimentary basin are based on overly simplistic stratigraphic architecture resulting in high geologic uncertainty. This study uses the relationship between environment of deposition (EOD)and connected pore volume to increase model accuracy by documenting standardized parameters across five reservoir classes (RC) with known inputs. The degree to which connected pore volume varies between RCs depends on which geologic controls are most influential.

Region-based modeling of the PRB, 7,000 km2 and over four km deep, employs 28,000 wells correlated basin wide with over 50 standardized stratigraphic surfaces. Sedimentation regions, each corresponding to an EOD, are defined by mapped thickness distributions tied to well-log shape and lithology cutoffs and then calibrated to paleogeographic and depositional system maps. A subset of 3,500 digital well log suites and petrophysics from 1,400 cored wells and outcrop studies provide rock property data for the model.

We compare 5 RCs and evaluate 17 standardized parameters in 5 categories (tectonics, facies, sedimentary bodies, paleogeographic region and stratigraphy) that governed sedimentation patterns. Stratigraphic signals in RC1-5, in order of decreasing depth, record the transition from more climate control to that of tectonics associated with foreland development. Climatic variations under Greenhouse conditions produced RC1, a long-lived Mississippian carbonate platform system (<500 m-thick; 12 m.y.) composed of laterally expansive vertically connected karst-bounded sequences. Secondary porosity follows depositional patterns and results in large areas of connected porosity. Brecciation provides zones of high permeability and preferred-facies fracture zones. Instead, under Icehouse conditions RC2 (<250 m-thick; 28 m.y.) shows more layering and interfingering of eolian erg, coastal dune, shelf and sabkha clastics with bands of basin-wide dolomitized clastics that record marine reworking of coastal dunes upon rapid transgression. This complex arrangement of environments produces potentially high, but more isolated connected pore volumes.

Alternatively, foreland development through the Cretaceous resulted in increasing subsidence rates, the onset of which recorded by thin (<50 m-thick) valley fill sandstones of RC3 during 2 m.y of latest Albian time. Gentle subsidence to the west and long-term transgression resulted in progressive onlap within a dendritic erosional surface and development of bay head deltas and tidal inlets during still stands. Subsequently, the deltaic wedge of RC4, half the 600 m-thick overlying RC5, records more erosion and bypass along forced regression surfaces in the more channelized delta front region (Cenomanian-Turonian), as evidenced by forced-regression erosion and basinal turbidites. The mature rapidly subsiding foreland of the Campanian influenced rapid accumulation of RC5 (half the 8 m.y. RC4) composed of shoreface sands and offshore bars encased in shelf mud. Minimal erosion along forced regression unconformities and thick mudstones separate three clastic wedges.

Preliminary results suggest that carbonate systems become much more heterogeneous under Icehouse conditions suggesting greater connected pore volume lies in Greenhouse carbonate platform systems. Furthermore, in the upper 3 RCs, foreland evolution phases were most influential, producing connected porosity restricted to valley fills, concentrated by significant erosion that gave way to significant subsidence. These patterns are fundamental to predicting connected pore volume from log data for depositional systems filling cratonic sedimentary basins

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado