Use of Sonic-Shear-Wave/Resistivity Overlay as a Quick-Look Method for Identifying Potential Pay Zones in the Ohio (Devonian) Shale

Abstract

Summary Until recently, there were few wells drilled in the Ohio (Devonian) shale because of the low profit potential and difficulty in identifying potentially productive zones using the conventional openhole logging suite (compensated density/sidewall or count-rate neutron/gamma ray logs). However, with the incentive of higher prices for oil and gas, drilling, activity in the Ohio shale has increased dramatically. Operators now are drilling deeper to search for productive zones in the shale below the Berea sandstone. As was the case with occasional attempts to produce from this same interval in the late 1960's, finding productive zones using the conventional openhole logging suite is often unsuccessful. Identification of possible productive zones in the Ohio shale is facilitated greatly by adding a borehole-compensated sonic log (BCSL) to the conventional logging suite. The additional information available from a sonic log can be used to create a quick-look method for identifying potential pay zones in the Ohio shale. This paper discusses the Ohio shale, problems associated with finding productive zones, the theory of using sonic data in addition to the conventional suite of logs, and the sonic-shear-wave/resistivity overlay technique. Introduction There has been much debate as to the source of gas from the Devonian shales. The main issue is: Does the as come from the pore space of natural fractures or from the shale matrix itself? This question was discussed in the Proceedings of the 1935 Devonian Shale Symposium (complete with production models and correlative data to prove either concept), and more recently by Smith et al. It is not the intent of this paper to answer that question or to advocate either concept but to review past and present methods for finding productive intervals within the Devonian shales no matter what the source of the hydrocarbons. In the final analysis, it is not the resolution of theoretical arguments that will meet our energy requirements but finding hydrocarbon zones and producing them. Recent production records prove that there are commercially producible hydrocarbons in the Ohio shale and that productive zones can be found. This paper shows how to find possible productive zones using the sonic-shear-wave/resistivity overlay technique. Geological Background The Devonian shales of eastern and central Ohio consist primarily of the Olentangy and Ohio shales in ascending order. The Cleveland shale pinches out against the Chagrin member as the formation is followed eastward. Thus the Ohio shale in eastern Ohio usually has only two components-the Chagrin and Huron shale submembers. This composition of the Ohio shale is discussed in this paper. Completion records of wells drilled in the Ohio shale often refer to silt-type stringer below the Berea sandstone and above the Onondaga limestone by a variety of names such as Gantz, Thirty Foot sand, Gordon sandstone, Gordon 4th, and Gordon 5th. However, the name "Gordon" originally was used to identify an Upper-Devonian sandstone of Pennsylvania and West Virginia-Gordon, as originally used, implies the presence of a clean sand with a peak gamma ray log comparable to the Berea sitndstone-characteristics not customarily found in the "Gordon" wells of eastern Ohio. In the Ohio shale, gamma ray logs consistently show a shaley pattern below the Berea sandstone and above the Onondaga limestone. Within the Chagrin shale members are siltstone stringers showing no sandstone characteristics. It is probable that the term "Gordon," as used in completion records of Ohio shale wells, refers to a siltstone within the Chagrin shale member. it fractured zone within the Chagrin shale member, or a combination of the two. JPT P. 638^