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A Case Study of Missed Opportunity: Effect of Fractures on Gas Reserves: Myrtle Springs Field (Smackover), Van Zandt County, Texas
Petrology can be a very effective tool in reservoir evaluation, especially where recovered reserves are greater than had been initially calculated. Myrtle Springs Field in Van Zandt County, Texas, produces gas from Jurassic carbonate grainstone deposits of the upper Smackover formation. The field is a faulted structure which produces gas from both sides of the fault. The producing interval is composed of ooids, pisoids, pelecypod fragments, grapestones, and fecal pellets that have been deposited as warm-water shallow-marine shoals, constructed as an interbedded pelletal-grainstone atop a pelletal-packstone seated on top of a pelletal-mudstone. During the marine regression at the end of Smackover time, these deposits were subjected to subaerial leaching for a considerable time resulting in fracturing and minor brecciation within the deposit. The physical processes that have formed the shoals of this field are not unique: in fact, many such shoals were formed in a similar manner along the Mexia-Talco fault zone. In calculating recoverable-reserves, log responses alone cannot be used to account for the effects of fracturing, but, combined with insights from petrological studies, a more realistic value can be calculated. Petrology takes into consideration a) the presence of stylolites and fractures which have enhanced both the porosity and the permeability of the reservoir; b) the large dolomite crystal content which has increased permeability and pore-size in the partially dolomitized limestone; c) the presence of moldic porosity; d) residual oil can be seen in thin-section samples especially in the fractured limestone; e) anhydrite and sparry calcite have subsequently filled many of the pores, especially below the oil-water contact; f) the low water saturation values. If known productivity indices from the field are plotted against petrological porosity and permeability-thicknesses, a best-fit line can be drawn to predict values for fractured zones.
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