Abstract

Pennsylvanian sandstones of the Mid-Continent region were deposited principally in deltaic, fan delta, and associated fluvial systems. Basic concepts of depositional processes and environments needed to explain the cyclic clastic and carbonate facies in the region were developed in the 1950s and 1960s as a result of Ideas generated from studies of Holocene analogues. The tectonic stability of shelves, platforms, and basins, and the structural style of fault-bounded source areas imposed distinctive geometries, vertical sequences, and areal distribution patterns on sandstone reservoirs.

Principal Mid-Continent sandstone reservoirs on moderately subsiding shelves and in moderately deep basins consist of lobate and elongate delta-front facies. In contrast, incised meanderbelt (point-bar) and valley-fill sandstones are the principal reservoirs deposited on exceedingly stable, shallow, Mid-Continent shelves where stream erosion removed much of the thin delta-front deposits. Distributary channel-fill sandstones are generally unimportant reservoirs except where they are superimposed on subjacent delta-front facies. Granite-wash sandstones and conglomerates, which were deposited in fan deltas and associated braided stream channels, constitute important reservoirs along the flanks of fault-bounded uplands.

On the Mid-Continent craton, broad, subtle structures tended to exert long-term, regional control on distribution of rivers and deltas. Paleotopographic variations resulting from an interplay of differential sand/shale compaction and local structural elements, however, controlled the intermediate distribution of these depositional systems. Locally, hydrologic and sedimentary factors dictated channel position and shifting of delta lobes.

As exploration moves into deeper parts of Mid-Continent basins, prediction of submarine fan reservoirs becomes important. Principal sources of deep-water sands were shelf and shelf-margin deltas. Consequently, deltaic sandstones are guideposts to deep-basin turbidite prospects as well as important reservoirs.