ABSTRACT: Depositional Facies, Reservoir Heterogeneity and Gamma-Ray Well-Log Signatures of an Estuarine Environment, Early Pennsylvanian, Southwestern Indiana

BARNHILL, M. L., and E. P. KVALE, Indiana Geological Survey, Bloomington, IN, and P. L. HANSLEY, U.S. Geological Survey, Denver, CO

During early Pennsylvanian time (late Morrowan to early Atokan), the eastern margin of the Illinois basin was dominated by a muddy, tidally influenced coastal plain that was forming in a tropical, equatorial climate. A study of more than 6,000 ft of core from 68 core holes, combined with data from outcrops at the Naval Weapons Support Center at Crane, Indiana, reveals details of vertical and lateral facies relationships of an estuarine-fill sequence.

Depositional environments included supratidal peat-forming swamps, supratidal to intertidal marshes, intertidal to subtidal sand and mud flats, subtidal sand-filled channels, and estuarine-funnel muds and sand shoals. Detailed subsurface correlation of core data reveals the extreme lateral and vertical discontinuity of these estuarine-fill facies (e.g., five ft thick coals pinch out in 600 to 800 ft). Tidally influenced point bars can be correlated laterally into intertidal sand flats and intertidal to supratidal marshes that overlie estuarine-funnel muds and sand shoals.

Porosity and permeability measurements along with scanning electron microscopy and petrographic analysis were conducted to determine differences in reservoir characteristics among the various sandstone facies. Extensive core control allows gamma-ray well-log signatures of specific estuarine facies to be determined. For example, wavy-bedded sandstone interpreted to have formed on intertidal to subtidal flats consists of alternating thin laminae (1 mm to 2 cm thick) of very fine grained sandstone and dark gray shale. This facies consists predominantly of sand and clay-sized particles, but because the gamma-ray tool averages the two, it displays this facies as intermediate between a "clean" sandstone and a shale. By cataloging the gamma-ray log responses of specific estuarine facies, a s t of models can be provided that may be helpful in recognizing estuarine facies in the subsurface where core control is not available. Estuarine sandstones are usually clean, well-sorted, and often encased in organic-rich shale therefore recognizing estuarine environments in oil-producing areas may be of economic importance.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)