Abstract: Lithostratigraphy and Hydrocarbon Potential of the Cambrian (Pre-Knox) Interval in the Conoco No. 1 Turner Well, Rough Creek Graben, Western Kentucky

David C. Harris

In 1992 an important deep exploratory well was drilled in the Rough Creek Graben of western Kentucky. The Conoco No. 1 Turner well, in McLean County, reached a total depth of 14,202 feet in Precambrian granite. The objective of this well was to test potential gas reservoirs in Cambrian-age pre-Knox Group carbonates and synrift sandstones. The well encountered no commercial hydrocarbons, but provided new data on the evolution and hydrocarbon potential of the Rough Creek Graben. Encouraged by the Turner well, Conoco has drilled two additional wells in the graben.

The Turner well penetrated over 4,000 feet of pre-Knox sedimentary rocks. This interval can be divided into four depositional sequences. The oldest sequence (1,900 feet thick) consists of fine- to coarse-grained, glauconitic lithic sandstones; thin green shales; and minor oolitic limestones. This interval is sandstone-dominated, and is interpreted to represent marine synrift deposits. Overlying the synrift clastics is an 1,150-foot-thick, heterolithic interval of fine- to medium-grained, glauconitic lithic sandstones; oolitic and fossiliferous limestones; and green shales. Smaller scale, shallowing-upward cycles can be recognized in this interval. These lithologies are interpreted as late synrift to early postrift deposits. The uppermost two sequences consist of a 600-foot-thick zone f dolomitized, oolitic grainstones overlain by a 350- foot-thick, shale-dominated interval composed of green shales, siltstones, and oolitic limestones. These intervals are interpreted as a marine post-rift deposit. The clastic rocks lack porosity in the Turner well (because of calcite and quartz cements), but may have reservoir potential in other parts of the basin. The oolitic dolostones contain dolomite cement, and also have no effective porosity. However, bitumen staining is present in the dolostones, indicating that oil moved through the interval.

The Conoco Turner well has proven that substantial thicknesses of synrift clastics and post-rift dolostones occur in the Rough Creek Graben, and that facies distribution is strongly tectonically controlled. Bitumen staining indicates that hydrocarbons were migrating in the basin, but prediction of effective porosity remains a problem in the search for Cambrian reservoirs in the Rough Creek Graben.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York