The San Joaquin basin (SJB) is a large asymmetric northwest-southeast trending sedimentary basin located in central California. Several of the most prolific oil fields in the continental U.S. are located within the SJB. Typical hydrocarbon production is derived from shallow heavy oil +/- 13 °API sand reservoirs and from moderately deeper lighter oil (25 to 35 °API) and gas sands as well as fractured siliceous shale reservoirs. Most heavy oil production is steam driven, resulting in compromised and variable water resistivity (Rw) values.

The dielectric logging service is used to provide a direct measurement of water filled porosity. This porosity, in conjunction with total porosity, is used to estimate the water saturation of the flushed zone (SXO). Consequently, the primary applications of the dielectric measurements in the SJB have been to resolve SXO in heavy oil steam drives.

This paper illustrates methods of reservoir analysis, proven in the SJB, that combine the dielectric data with standard triple-combo data to evaluate these complex reservoirs. With the recent downturn in commodity prices, this method has proven to be a cost effective alternative for evaluating this type of reservoirs.

The San Joaquin Basin

The San Joaquin basin (SJB) is a large asymmetric northwest-southeast trending sedimentary basin located in central California (Fig. 1). The US Geological Survey (USGS), using a geology based assessment methodology, estimated the following mean values:

  • 1.7 trillion cubic feet of undiscovered natural gas

  • 393 million barrels of undiscovered oil

  • 86 million barrels of natural gas liquids (NGL)

According to the USGS (1), "Well over half of both the mean undiscovered gas resource (62 percent, 1 TCFG) and the mean NGL resource (67 percent, or 57 MMBNGL) is in the Deep Fractured Pre-Monterey Assessment Unit of the Eocene-Miocene Composite Total Petroleum System. Of the mean 393 MMB of total undiscovered oil, about 328 MMBO (83 percent) is estimated to be in five assessment units of the Miocene Total Petroleum System, and 46 MMBO (12 percent) is estimated for the Eocene West Side Fold Belt Assessment Unit of the Eocene Composite Total Petroleum System.

" Nearly 31%, or 121 MMB, of the total mean undiscovered oil resource in the San Joaquin Basin Province is estimated to be in the Central Basin Monterey Digenetic Traps Assessment Unit.

"Potential reserve growth in existing oil and gas fields in the San Joaquin Basin Province was calculated during our study. Based on the analyses of recovery efficiencies and remaining oil-in-place, we estimated that another 3.5 billion barrels of oil may be added to reserves in existing fields. Most additional reserves may come from improved recovery in diatomite reservoirs of the Miocene Monterey Formation. This estimate of potential reserve growth suggests that most oil in the future may come from existing fields rather than from new oil field discoveries in the San Joaquin Basin."

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