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Combined geochemical study and traditional system modeling of the Salinas Basin

Salinas Basin geochemistry
Graduate student Tess Menotti is combining traditional basin and petroleum system modeling of the Salinas basin, California, described here, with a geochemical study of the area.  The reliability of thermal history calibration using vitrinite reflectance (Ro) or Tmax pyrolysis depth profiles is improved with a higher density of calibration data. The Texaco Shell NCT-1 well, on which burial history modeling was based, has only four Tmax data points, measured in the 1980's. For this reason, as well as the paucity of thermal maturity data of modern vintage in general, Tess sought sources for new data, particularly for the deepest part of the basin in the Hames Valley Trough. 
 
Although the California Well Sample Repository houses several Salinas Basin cores, none of the samples still available are of useful depths for a complete burial history analysis. However, an operating outfit in the Salinas basin, Trio Petroleum, provided Tess with 26 well cutting samples from 150-foot depth intervals from 5490 to 9390 feet MD. Tess is using these well cutting samples in two ways: 

  1. Rock Eval Pyrolysis by GeoMark Ltd, primarily for Tmax values.
  2. Biomarker analysis of bitumen extractions from cuttings.
There are analytical hazards associated with relying on well cuttings for a thermal maturity depth profile (Peters and Cassa, 1994; whole core, or sidewall core is preferable), and these are taken into account in assessment of the quality of the Ro and Tmax depth profiles.
 
Geochemical signatures of crude oils can also assist in constraining thermal exposure of a source rock at the time of generation of that oil (Peters et al., 2005). This is less applicable to burial history analysis, as it is impossible to tell precisely from which depth the oils have expelled from the source rock. However, oil maturity can provide a constraint to corroborate modeled burial histories. With the aid of multiple Salinas basin petroleum companies and the USGS, Tess collected crude oils from the five fields, as well as oil from a recently drilled well on the flank of the Hames Valley Trough. Additionally, Tess collected three tar seep samples from outcrops in the southern Salinas basin. From initial inspection of the produced oil data, the diamondoid abundance is very low in all samples (<2.7 ppm), indicating that these oil samples experienced insufficient heat to be thermally cracked (e.g., Dahl et al., 1999), and are of normal maturity. Based on the ratio of 28,30-bisnorhopane to hopane (corrected for biodegradation using trisnorhopane and 25-norhopane), there is a spatial trend in thermal maturity of the oils, with increasing maturity in the accumulations from south to north. This observation provides calibration for migration histories in future 3D basin models of the Salinas basin.
 
Future work will include additional biomarker analysis of oils provided by Venoco, Inc., as well as additional tar seep samples from the western side of the Reliz-Rinconada Fault (RRF).