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Geologic Evaluation And Remaining Potential Of An Abandoned Oil Field: The Santa Margarita (Reef Ridge) Sandstones Pool Of The San Emidio Nose Field, California.


The Santa Margarita (Reef Ridge) Sandstones pool of the San Emidio Nose Field is located in the Southern San Joaquin Basin, California. It was discovered in 1958 by Richfield Oil Corporation and developed by 18 subsequent wells that produced over 10 MMBO. The Santa Margarita pool was active until 1981, when the last producer was plugged and abandoned. The accumulation is formed by the pinch-out of three different sand units (Sands #1, #3, #4) against the San Emidio paleohigh. This pinch-out as well as a detailed structure map at the top of each individual reservoir has been interpreted in detail using both 3D seismic and all the well data. Based on this interpretation the boundaries of the pool are better understood and the field could potentially be extended to the east. Approximately half of the well completions commingled the three reservoirs. However, Sand #1 is thought to have been the most prolific because of its better reservoir quality with permeabilities ranging from 10 to 300 mD. Sands # 3 and #4 correspond to thinner and lower quality sandstones (1 to 20 mD) which are interpreted to have less recovery than Sand #1. Cumulative production and initial oil rate maps for the Santa Margarita pool show a good correlation with respect to the thickness and structure at the top of Sand #1 confirming that the uppermost reservoir is likely the main contributor to the historical production. A 2014 exploration well targeting deeper intervals in the San Emidio Nose Field encountered better gas shows, higher oil saturations and higher pressures in Sands #3 and #4; it also found that the resistivity values of these two intervals remained the same magnitude as logged in the original wells. In contrast, Sand #1 showed a significant decrease in resistivity with respect to the original values, dropping from 50-80 ohm-m down to 10-20 ohm-m. These data support the interpretation that the recovery from Sands #3 and #4 was poor in the original development, and provide significant development potential for the Santa Margarita accumulation.