INTEGRATED ICHNOLOGY, SEDIMENTOLOGY, AND SEQUENCE STRATIGRAPHY OF A LOWER PENNSYLVANIAN OIL PLAY, SOUTHERN OHIO

The use of trace fossils has grown exponentially over the past 60 years. Trace fossils can help reveal a wide range of paleoecologic parameters such as salinity, oxygen, rates of deposition which are not readily apparent by other criteria, and which can help identify systems tracts and their boundaries. Their occurrence in paralic strata that lack body fossils makes them especially important. This study is based on analysis of 23 outcrops, 10 cores, and 90 gamma ray and density logs in and near the Greasy Ridge oilfield.

The oilfield is an isolated area of production in the Lower Pottsville Group of southern Ohio. The reservoir sandstone occurs in the younger of two parasequences between the No. 1 and No. 2 coals. The lower parasequence is 14 m thick beginning at the top of the No. 1 coal and contains 4 facies. The basal facies (1, prodelta) is a dark gray fissile shale with sideritic claystone bands and Lingula. Facies (2, distal mouth bar) consists of fissile shale with discontinuous siltstone laminae and very small horizontal burrows (Planolites). It is overlain by facies 3 (proximal mouth bar) consisting of interstratified dark gray shale and light gray very fine sandstone containing large diverse burrows including Teichichnus, Phycodes, Planolites, Zoophycos, and Lingulichnus. Facies 4 (abandoned mouth bar) consists of thoroughly bioturbated, argillaceous fine sandstone with sideritized burrows and nodules. Facies 1-4 indicate progressive shoaling and improved oxygenation from offshore to nearshore environments. The ichnogenera from facies 3 represent a reduced (‘depauperate’) Cruziana ichnofacies which commonly occurs in nearshore and estuarine heterolithic facies and fits a tide-influenced distal mouth bar interpretation. Facies 4 represents still higher energy and dissolved oxygen, and an abundant infaunal biomass, along with slow sedimentation, and normal marine salinity. The upper parasequence is 17.6 m thick. The lower 7.6 m contains the same distal and proximal mouth bar facies in the lower parasequence. The upper 10 m includes the oil sand and consists of fluvial, tidal, and abandoned distributary channel-fills capped by the No. 2 Coal. Deposition occurred in a tide-dominated delta that prograded southwestward into a narrow, restricted seaway that paralleled the axis of the Appalachian foreland basin.