--> --> Identifying Reservoir Development in Carbonate Rocks by Qualitative Analysis of the Spontaneous Potential Curve: Example from the Caddo Limestone of Western Jack County, Texas

AAPG Southwest Section Annual Convention

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Identifying Reservoir Development in Carbonate Rocks by Qualitative Analysis of the Spontaneous Potential Curve: Example from the Caddo Limestone of Western Jack County, Texas

Abstract

A significant portion of the world’s hydrocarbon reserves are found in carbonate reservoirs. North and West Central Texas have seen oil and gas fields produced from multiple limestone formations. Many of the early discoveries were made when logging suites consisted of only a resistivity log with a spontaneous potential curve. The spontaneous potential curve also referred to as the SP Curve, can detect permeable beds. The only requirements are a conductive well bore fluid, a porous and permeable bed sandwiched between low or impermeable beds, a difference in salinity between the bore hole fluid and the formation fluid and lastly, the fluid salinity needs to be the same within the permeable bed and the upper and lower bounding, low or impermeable beds. The Caddo Limestone produces hydrocarbons in Jack County, Texas. Many of the wells drilled through the Caddo, were drilled prior to the advent of the density log. Many of the recently drilled wells only release, to the public, the induction log, containing, a SP curve and possibly a Gamma Ray. Therefore, in many cases, the only publicly available log is a resistivity log with spontaneous potential curve. The SP curve can be qualitatively analyzed as being poor, fair, good or excellent depending on the deflection of the curve in the formation of interest. Little to no deflection is considered poor and is referred to as the baseline. As the SP curve deflects from the baseline the reservoir class will change from poor, to fair, to good and in extreme cases as excellent. The SP curve can be compared with the SP in other well logs, by normalizing the SP log grid scale. By using the same grid scale for all well logs, the SP can be directly compared. The magnitude of the grid units is immaterial, as long as the same unit is used for all well logs. A prospect lead was identified in west central Jack County by integrating the Caddo Limestone structure, isopach and reservoir characterization maps. Six wells were drilled on the prospect and have produced a combined total of 285,135 BO and 868,111 MCF since July 2008.