Abstract

One-dimensional basin modeling activities using the program BasinMod have focused on analysis of the Elk Hills 934-29R well in the Naval Petroleum Reserve, the deepest well (24,442 feet) in the San Joaquin Valley. The well has excellent equilibrated downhole temperature profiles based on continuous temperature logging and a good vitrinite reflectance (Ro) profile that provide for calibration of the paleo-heat flow. A generalized paleo-heat flow model for the southern San Joaquin Basin is provided by model studies of lithospheric plate interactions which predicts the thermal consequences of the complex plate interactions affecting the southern San Joaquin Valley. The results of the 1-D modeling study of Ek Hills 934-29R demonstrate that the downhole temperature profile observed in the well is not in equilibrium with observed Ro data. This phenomenon is also observed at North Coles Levy Field to the north and suggests that econonic basement may be much deeper than previously believed.

It appears that the elevated temperature profile may be due to episodic release of geothermal fluids that began within in the last 30,000 years. If correct, our interpretation suggests that all reservoirs shallower than approximately 17,000 feet are prospective. This is much deeper than any of the reservoirs presently being produced in the San Joaquin Valley, which are generally shallower than 10,000 feet.

A fluid inclusion study of vein carbonate samples taken from core recovered from between 12,000 and 24,000 ft in the EH 934-29R well was completed. All of the samples contain petroleum fluid inclusions that range from light oil in the shallowest samples, through condensate in the intermediate samples, to wet gas in the deepest sample, providing evidence that hydrocarbon migration has occurred throughout this deep section.

Well History of EH934-29R

Location:	Naval Petroleum Reserve #1 at Elk Hills in the southern San Joaquin Valley west of Bakersfield and due north of Taft (Figure 5) . It is one of 3 deep wells drilled by the Department of Energy during the late 70's and 80's.
Purpose:	To explore the potential for deep hydrocarbon reserves.
Drilling started:	May 30, 1985
Drilling ended:	August, 1987
Drilled Depth:	24,426 feet -- It is the deepest well ever drilled in California.
Maximum Temperature:	431oF at total depth.
Results:	Oil was produced from several deep intervals down to 18,000 feet with some shows as deep as 23,000 feet. Gas shows were found in all sandstone units and production tests recovered gas in varying amounts to depths near 24,000 feet.

Stratigraphic Column

The stratigraphic column (Figure 6) summarizes the geologic data obtained from Elk Hills 934-29R and shows the formation boundary ages used in modeling the geohistory of the well. This is the only well in the San Joaquin Valley to penetrate sediments of Cretaceous age at this depth. Hatched boxes represent zones of current oil production at Elk Hills. Gas and oil shows are those found in well 934-29R during drilling or from production tests after completion of the well.

Source-rock Maturity

Two observations are frequently used to determine the potential for oil and gas reserves in a basin: vitrinite reflectance (Ro) and downhole temperature measurements taken during well logging operations.

Source-rock maturity is estimated by measuring light reflectance from Vitrinite, a thermally altered component of plant material. As plant material is exposed to higher temperatures vitrinite reflectance increases. Therefore, vitrinite reflectance measures the amount and intensity of heat experienced by the plant material during its burial history.

Downhole temperatures measured after drilling are frequently used to estimate sourcerock maturity (by empirical correlation based on paired data from other wells in the basin) when Ro data are not available. Because temperature data are collected during well logging, shortly after drilling and before well-bores temperatures have had sufficient time to equilibrate with the true formation temperatures, temperature data are in most cases unreliable estimates of in-situ formation temperatures.

EH934-29R temperature data sets are unusual in that they were collected 1 - 2 years after drilling ended and represent a "true equilibrium temperature" profile.

Therefore the vitrinite reflectance and temperature sets derived from this well provide an excellent opportunity to more realistically calibrate the thermal history at Elk Hills. These data are shown in Figures 7, 8 and 9 and are further discussed in the following discussion

Thermal History

The thermal history of a well can be determined (using Platte River Associates BasinMod program) from reconstruction of the burial and structural history of a well. Required data are: formation ages, thicknesses, lithologies, porosities and thermal properties. In addition, models of the basal heat flow and surface temperature histories for the southern San Joaquin Valley are required. Fortunately, the San Joaquin basin and its tectonic history has been studied extensively and the data requirements are satisfied. Of particular importance to developing a useful thermal history for this well is the understanding of the heatflow history of the region.

The heatflow model used in this study involves the existence of a low constant heat flow until 25 million years ago at which time heatflow began to increase to present day values. The existence of this lower heat flow in the late Cretaceous and early Tertiary has been postulated based on structural studies (Dickenson and Snyder, 1979) and plate subduction model studies that calculate the heatflow consequences of Farallon Plate subduction beneath the North American Plate (Lachenbruch and Sass, 1980; Zandt and Furlong, 1982; Furlong, 1984; Heasler and Surdam, 1985; Dumitru, 1986). The relatively recent rise in heatflow is a result of cessation of subduction and the northward migration of a thermal pulse associated with the movement of the Mendocino Triple Junction (MTJ). The thermal pulse is associated with what has been called a "slabless window" beneath the North American Plate where the Farallon Plate used to exist but which subsequently migrated northward along with the MTJ. Hot aesthenosphere subsequently filled the void below the thin (~20km) North American Plate. Recent Apatite Fission Track studies by Dumitru (1988, 1989, 1990) have confirmed the existence of low temperature gradients (~9oC/km) in the late Cretaceous and early Tertiary. These temperature gradients are consistent with the early, low heat flow value calculated in the heat flow model studies cited and used herein.

The heatflow model described above was adjusted to match downhole temperature data in EH934-29R and vitrinite reflectance values were calculated for the resultant heat flow model (Standard Model). Figure 7 shows a plot of predicted downhole temperatures and vitrinite reflectance as a function of depth in the well. Note the close match of the calculated temperature curve with a subset of the downhole temperature data indicating a successful calibration of the heatflow model with the observed temperature data . However the predicted Ro maturity curve is dramatically higher than the observed Ro data indicating that the vitrinite is not in equilibrium with the present day temperature field. In addition it suggests that the present day temperature field must have existed for a relatively for a relatively short as vitrinite is known to equilibrate relatively rapidly.

A new heat flow history model (Proposed Model) developed that would match both downhole temperature and Ro data. This was done by first calibrating the heatflow model against the Ro data and then iteratively adjusting the magnitude and duration of a recent heat pulse until the present temperature profile was predicted without significantly changing the Ro prediction. Figure 8 shows that both calculated Ro and downhole temperature curves now show a close correspondence to the data.

Figure 9 compares the temperature histories at the base of the well for the Standard and Proposed heatflow model. In the Proposed Model the basal temperature does not rise as rapidly near present day as in the Standard Model until about 30,000 years ago when the temperature begins to rise rapidly to the present day temperature.

The Proposed Model indicates that within the last 30,000 years there has been a dramatic increase in heat flow. We suggest that this increase in heatflow is probably related to increased seismic activity in this period. The seismic activity probably reflects increased movements along faults in the area causing migration of hot fluids from the deep basin to basin margin structures close to the San Andreas Fault, such as those found at Elk Hills.

Supporting Data

Fluid inclusion data from core samples containing calcite-rich veins have been used to estimate temperatures of geothermal fluids. The depth-temperature plot in Figure 10 shows that measured fluid inclusion temperatures approach the presently observed downhole temperatures and above the temperature curve that is in equilibrium with the observed vitrinite reflectance data. Because fluid inclusion temperatures are minimum temperature estimates, these data are reasonably consistent with the present day temperature profile suggesting that the fluids were trapped at times within the last 30,000 years depending on where each data point lies between the two curves.

It is also observed that many fluid inclusions in vein calcites were found to contain abundant oil and gas fluids indicating that the crystals were formed while hydrocarbon fluids were migrating to the Elk Hills reservoirs. This further implies that hydrocarbons are probably intermittently migrating to reservoirs in Elk Hills at the present time.

Observations from water wells close to the field and along structural strike show high temperature and salinity anomalies. Surface water temperature anomalies of 4 to 5oF above background values of 69 to 70oF were observed at the southeast end of the field in line with a major axial fault and around the periphery of the northeast side ofthe field. Conductance values in these same wells were elevated to values of 3000 to 8000 micromhos as compared with background values of approximately 300 micromhos when compared to regional averages. These data suggest that hot geothermal fluids are migrating to near surface aquifers at the present time.

The implications of this study are that the likelihood of deeper hydrocarbon bearing reservoirs at Elk Hills is high if reservoir quality is satisfactory. In addition, if Elk Hills is a representative analogue for the structurally complex fields along the western side of the San Joaquin basin, then disequilibrium between present day temperatures and thermal maturity of the sediments is more likely than not. Therefore using bottom-hole temperature data alone as an indicator of the depth of the base of the "oil window" will lead to the erroneous elimination of deeper targets, and thus downplay the significant deep exploration potential of the basin.

Acknowledgments

Elk lells Naval Petroleum Reserve Management was instrumental in providing access to reports, data and cores. We particularly thank, George McJannett of DOE and Tim Reid of Bechtel for their support.

Selected References

Dumitru, T. A. (1986) Plate Tectonic Controls on Time-Varying Geothermal Gradients in the Great Valley Forearc Basin, Califomia: Fission Track Analysis and Computer Modeling, M. A. Thesis, 296 PP., University of Texas at Austin.

Dumitru, T. A. (1988) Subnormal Geothermal Gradients in the Great Valley Forearc Basin, Califomia, during Franciscan Subduction: A Fission Track Study, Tectonics, 7, 1201-1221.

Dumitru, T. A. (1989) Constraints on Uplift in the Franciscan Subduction Complex from Apatite Fission Track Analysis, Tectonics, 8, 197-220.

Dumitru, T. A. (1990) Subnormal Cenozoic Geothermal Gradients in the Extinct Sierra Nevada Magmatic Arc: Cosequences of Laramide and Post-Laramide Shallow-Angle Subduction, Journ of Geophs. Res., 95, B4, 4925-4941.

Fishbum, M. D. (I 990) Results of deep drilling, Elk Hills Field, Kern County, Califomia. In Structure, Stratigraphy and Hydrocarbon Occurrences of the San Joaquin Basin, California, (editors J. G. Kuespert and S. A. Reid), Pacific Section, Society of Econornic Paleontologists and Mineralogists, Bakersfield, CA., pp. 157-168.

Fischer, K. J., Heasler, H. P. and Surdam, R. C. (1988) Hydrocarbon Maturation Modeling of the Southern San Joaquin Basin, California, in Studies of the Geology of the San Joaquin Basin, (editor S. A. Graham), Pacific Section, Society of Economic Paleontologists and Mineralogists, Los Angeles, CA, pp. 53-64. Fisher, J. B. and Boles, J. R. (1990) Water-rock interaction in Tertiary sandstones, San Joaquin basin, Califomia, U. S. A. - Diagenetic controls on water composition. Chem. Geol. 82, 83- 101.

Furlong, K. P. (I 984) Lithospheric Behavior with Triple Junction Migration: An Example based on the Mendocino Triple Junction, Physics of the Earth and Planetary Interiors, 36, 213-223.

Heasler, H. P. and Surdam, R. C. (1985) Thermal Evolution of Coastal Califomia with Application to Hydrocarbon Maturation, Bull. Amer. Assoc. Petro. Geol 69(9), 1386-1400.

Lachenbrach, A. H. and Sass, J. H. (1980) Heat Flow and Energetics of the San Andreas Fault Zone, Journ. of Geophys. Res. 85, 6185-6223. Maher, J. C., Carter, R. D. and Lantz, R. J. (1975) Petroleum Geology of Naval Petroleum Reserve No. 1, Elk Hills, Kern County, Califomia. U. S. Geol. Surv. Prof. Paper 912, United States Government Printing Office, Washington.

Nicholson, G. E. (1990) Structural overview of Elk Hills. In Structure, Stratigraphy and Hydrocarbon Occurrences of the San Joaquin Basin, California, (editors J. G. Kuespert and S. A. Reid), Pacific Section, Society of Economic Paleontologists and Mineralogists, Bakersfield, CA., pp. 157-168.

Reid, S. A. (1990a) Elk Hills Field overview, In Structure, Stratigraphy and Hydrocarbon Occurrences of the San Joaquin Basin, California, (editors J. G. Kuespert and S. A. Reid), Pacific Section, Society of Economic Paleontologists and Mineralogists, Bakersfield, CA., p, 131-132.

Reid, S. A. (1990b) Trapping characteristics of Upper Miocene turbidite deposits, Elk Hills Field, Kern County, California, In Structure, Stratigraphy and Hydrocarbon Occurrences of the San Joaquin Basin, California, (editors J. G. Kuespert and S. A. Reid), Pacific Section, Society of Economic Paleontologists and Mineralogists, Bakersfield, CA., p. 133-140.

Surdam, R. C., Fischer, K. J. and Heasler, H. P. (1988) Hydrocarbon maturation modeling of the Southern San Joaquin basin, California: In Studies in the geology of the San Joaquin basin, (editor S. Graham), Pacific Section Soc. Econ. Paleontologists Mineral. Spec. Pub. pp. 53-64.

Sweeney , J. J. and Bumham A. K. (1990) Evaluation of a Simple Model of Vitrinite Reflectance based on Chemical Kinetics, Bull. Amer. Assoc. Pet. Geol. 74(10) 1559-1570

Zandt, G. and Furlong, K. P. (1982) Evolution and Thickness of the Lithosphere beneath Coastal Califomia, Geology, 10, 376-381.

Zieglar, D. L. and Spotts, J. H. (1984) Reservoir and source bed history in the Great Valley, California. In Petroleum Geochemistry and Basin Evaluation (editors G. Demaison and R. J. Murris) Amer. Assoc. Petrol. Geol. Mem. 35, 193-204.