Abstract

A renewed focus has been emphasized for Petroleum Geologists to identify unconventional reservoirs capable of sustaining commercial resource plays throughout the continental US. In the Midland Basin and Eastern Shelf, an organic rich Pennsylvanian shale has been identified and targeted for a resource play nicknamed the ‘Cline’ formation. The ‘Cline’ formation consists of a series of porous hydrocarbon saturated turbiditic silts interbedded in organic rich ductile shales and detritus limes deposited in a low stand anoxic basin trending northeast to southwest. Historically, the ‘Cline’ – also known as the ‘Black Shale’ on the Eastern Shelf – has produced randomly over an eight county area from Fisher County to the northeast, Irion County to the south, and Midland/Martin County to the West. On the eastern portion of the Midland Basin, thirty eight (38) wells tested and produced from the Cline Formation. Nineteen (19) of these wells produced the Cline formation in ‘isolation’ averaging 43,805 BO and 83,224 MCFG per well with two historical wells still producing presently (Chapman Denton # 1 and Cobra Guitar 7 #1). The Cline is typically 300’ thick but can be found in basinal lows as thick as 800’. Mud logs consistently record drilling breaks and strong gas kicks across the Cline interval. The hydrocarbon saturated turbiditic silts are characterized on logs by an extremely hot gamma ray typically greater than 125 API and a positive resistivity spike corresponding to a porous density kick. Cores taken from the Cline show porosities ranging from 6 to 16% in the silt pays, 3 to 6 % in the organic rich ductile shales, TOC content of 4 to just over 10 % in the silt pays – type III mature gas prone, and gas desorption rates average 112.3 scf/ton ranging as high as 153.7 scf/ton. Permeability’s typically range from 300 to 600 md but whole cores exhibit perms as high as .1 md. XRD mineral analysis indicates the hot silts to have a silica content as high as 40% (typically 25 to 30%) with overall clay contents ranging as high as 55%. The Cline produces by means of hydrocarbon expulsion into existing fractures from ΔP across the length of the fracture. Historical production curves follow two distinct patterns; 1) high rate/short life production – such as the Fasken Costilla Guitar 5 #1 16,000 BO in 7 months or the Texas National Tom # 1 173,093 BO in 4 ½ years – 2) low rate flat long life production – such as Berry & Hancock Houston Poe # 1 108,847 BO 349,939 MCFG in 25 years of production. All historical producers (with the exception of the Cobra Guitar 7 #1* – small 10,000 # frac one month after being put on production with acid) were completed with acid treatments indicative of encountering natural fractures primarily induced from localized tectonic stresses or encountering thin porous detritus limes depositionally associated with overlying or underlying hydrocarbon saturated silts allowing the lime to act as a ΔP ‘conduit’ for production to the wellbore. Core results and historical producers (such as the Chapman Denton #1 or the Continental Oil Settles # 1), verify that the hot silt channels have enough conventional permeabilities – i.e. >0.1md – to sustain low flow rates with commercial production to the wellbore. Applying modern frac techniques, developed in major shale resource plays ongoing throughout the continental US, to the Cline should allow the Cline to add significant reserves to the ‘Wolfberry’ play in the Midland Basin or stand alone as a new commercial resource play in ‘sweet spots’ across the Midland Basin and Eastern Shelf.