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The AAPG/Datapages Combined Publications Database
West Texas Geological Society
Abstract
Rubble In The Hole: Much Ado About Nothing
Abstract
Coring in a well in the southern portion of the Bryant G Devonian Unit (western Midland Co., Texas) recovered a short interval of black cobbles sandwiched between the normal buff to light gray platform carbonates of the Thirtyone Formation. The radically different lithologies represented by these cobbles, combined with the presence of a “kick” in the gamma ray log, suggested the presence of a major sequence boundary in this heretofore-uncored interval. On the other hand, problems encountered during drilling, the presence of slickensides and shocked fabrics in some of the cobbles and exotic mineralogy of some samples suggested that the rubble in the core was derived from a fault zone. However, no faulting is evident in the seismic data for this portion of the field. The most probable explanation for these unusual lithologies within the cores was arrived at with the identification of foraminifers of the genus Tetrataxis in one of the limestone clasts. How did cobbles of diverse Mississippian lithologies end up in Devonian cores? They probably fell into the wellbore during a coring trip, from a faulted interval in the overlying Woodford Shale or younger section.
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