LARGE ACCUMULATIONS OF OIL LIKELY BENEATH THRUST FAULTS IN EAST TEMBLOR RANGE AREA

Russell R. Simonson
Geologist and Engineer
Glendale, Calif.

The southern part of the San Joaquin basin is one of the most prolific oil producing areas of the U.S. It accounts for 75% of California's annual production or 248 million bbl/year of oil.

A large portion of this production is from "giant" fields (over 100 million bbl) or "supergiant" trends that will ultimately each produce 1 billion bbl or more of oil.

These prolific fields are usually associated with structural highs that are updip from depocenters that contain thick sections of excellent source rocks. Most of these large accumulations are on anticlines with fault, stratigraphic, and structural traps.

Anticlines in the western San Joaquin basin are responsible for most of the large fields in the area (Fig. 1).

The more important ones are the (1) Coalinga-Kettleman, Lost Hills anticline (1 2.5 billion bbl), (2) the North and South Belridge trend (1 billion bbl), (3) Elk Hills-Cymric high (2.3 billion bbl), (4) Buena Vista Hills anticline (700 million bbl), and (5) Belgian anticline (70 million bbl).

Midway-Sunset field (6), which is on the east flank of the Sub-Thrust anticline, is both a stratigraphic and structural accumulation containing 2.3 billion bbl.

UNRECOGNIZED ANTICLINE

The Sub-Thrust anticline (7) is the next structural trend west of the Midway-Sunset syncline and the Buena Vista Hills anticline.

It has not been previously recognized because of its position below the Cree thrust fault.

The surface geologic map (Fig. 2) shows the Cree fault bringing McDonald shale (Late Miocene, Lower Mohnian stage) over Antelope shale (Late Miocene, Upper Mohnian Stage). This overthrust relationship is confirmed in the subsurface as shown on cross section A-A' where the fault was encountered in three wells (Fig. 3).

The Phelps anticline is a closed structure in the hanging wall of the Cree thrust fault.

it was first drilled in 1929 by the Getty Oil Co. 1 Phelps well, which bottomed at 3,504 ft in steeply dipping, fractured, oil-stained McDonald shale above the sole of the Cree fault.

The Hudbay Oil Co. 1 Cree drilled in 1976 was the first well to reach the Cree fault. After encountering almost 1,000 ft of steeply dipping strata in the hanging wall, this well penetrated the Cree thrust at 7,300 ft and bottomed at 7,700 ft in low west dipping McDonald shale on the west flank of the SubThrust anticline.

OIL ON DRILL STEM TEST

The Frantzen Oil Co. 1 Cree drilled in 1983 found the Cree thrust fault at 5,865 ft.

It was not directed but allowed to drift to the east, which is up dip to the 25-35 westerly dipping section on the west flank of the SubThrust anticline.

This well had significant oil and gas shows in the McDonald shale and Devilwater-Gould sections below the Cree fault from 5,865 ft to T.D. of 9,502 ft.

Two formation tests were run. The deeper one at 9,198-9,502 ft failed. A test of the McDonald shale at 8,2478,51 0 ft resulted in a recovery of 1,200 ft of clean, 34 gravity oil and a 40 Mcfd rate gas flow.

A shut-in bottom hole pressure of 3,430 psi is normal hydrostatic pressure common to most producing structures in the southern San Joaquin Valley. The partial equivalent of this interval has produced more than 8,000 bbl of oil in the Arco Oil & Gas Co. 1A Cree Fee well from 8,220-40 ft.

Many shallow wells in the Temblor Range have had oil sands that have not produced either because of their high elevation or because they are cut off from valley hydrostatics because of barrier faults. This does not seem to be the case however, for deeper wells near the SubThrust high.

ARCO'S RESULTS

Arco drilled the 1A Cree Fee in 1985 to a T.D. of 14,452 ft.

This well encountered the Cree thrust fault at 5 915 and drifted easterly in the west dipping section to 8,230 ft, where it crossed the axis of the Sub-Thrust anticline.

The well then drifted westerly in penetrating east dipping strata on the east flank of the Sub-Thrust structure. The Arco well did not reach the Oligocene Oceanic or Eocene Point of Rocks sands but bottomed in Santos shale of Early Miocene, Zemorrian stage at T.D.

In outcrop west of the MidWay Peak fault there is 200 ft of Middle Miocene turbidite sand that is oil stained. This sand was not present in the Arco well but could be productive to the north if the channel crosses the SubThrust anticline.

UNTESTED SECTIONS

Fig. 6 is the type log for Belgian Anticline field some 5 miles north of cross section A-A'. A section similar to this remains to be tested at depth in the Sub-Thrust anticline.

In addition there could be a Cretaceous section that had shows in the deep test (Well No. 934-29R) in Elk Hills field, which also might be tested.

In the Gonyer Canyon area some 15 miles to the south there is evidence that conditions are similar to those in the Cree area. Here a large subthrust structure is present below a shallow closed surface structure as shown on cross section B-B'.

The Arco No. 78 well goes from Late Eocene strata back into Early Miocene at the thrust fault contact. This thrust is probably the Cree fault as shown on cross section A-A' to the north.

The dipmeter results on this well both above and below the fault indicate that it is located on the west flank of the Sub-Thrust anticline. East dip is well established by many wells drilled on the west flank of the Midway-Sunset syncline, including the Terra Resources Inc. 1-33 Federal well.

As is visible on cross section B-B', this well migrated updip to the west because it encountered consistent easterly dips. The axis of the SubThrust anticline is apparently located between these two wells and below the surface anticline as depicted on cross section B-B'.

The relationship of a structure in the hanging wall of a thrust fault being directly above a sub thrust anticline can be observed in Venture, San Miguelito, Aliso Canyon, and possibly other fields in the state.

It is believed, however, that this relationship cannot be entirely relied upon to determine the location of folds below thrust faults. Their exact position must be confirmed by detailed seismic, core drilling, subsurface work, or other methods.

Fig. 5 is a projection of the Sub-Thrust anticline between sections A-A' and B-B', a distance of 15 miles. Deep drilling as well as dependable seismic and subsurface work will be needed to more adequately define the location of this structure.

SUB-THRUST OBJECTIVES

Fig. 7 is a summary of objectives to be tested on the Sub-Thrust anticline. They are as follows:

The McDonald shale produces 6-10 b/d in the Arco 1 A Cree Fee well from 200 ft of fractured McDonald shale. There is a 2,500 ft interval between the perforated zone and the Cree fault that should be tested.
This shale section should also be an ideal candidate for horizontal drilling to expose more fracture patterns to production. Underbalanced drilling with low water loss mud or foam might result in good producers.
Some of the newer methods of hydrofracturing might also increase oil production and recovery.
The 200 ft Middle Miocene sand, which is oil stained in outcrop west of the Midway Peak fault, may be present and productive north of the Cree wells. A turbidite sand crossing the axis of the fold could trap oil similar to the accumulations at Elk Hills, Yowlumne, and Land-slide fields.
The Arco well had gas shows in the Temblor-Vaqueros section that were not tested.
Oil sands of this age in the abandoned Gonyer field apparently were not productive because of lack of hydrostatic drive at their high topographic elevation in the hanging well above the sole of the Cree fault. These sands might be productive if found on structure at greater depths under normal hydrostatic conditions.
Below the section reached in the Arco Cree well are untested objectives that may be productive in the Sub-Thrust anticline. They are the Oceanic and Point of Rocks sands, which are oil and gas producers in Belgian Anticline field some 5 miles to the north, and possible reservoirs in the Middle Eocene and Cretaceous section.

It is believed that significant accumulations will be found under thrust faults in the eastern Temblor Range. Similar conditions such as access to source rocks, normal hydrostatic gradients, favorable structural and stratigraphic traps, and other factors that were instrumental in forming such major fields such as Elk Hills, Buena Vista Hills, and Midway-Sunset should be present in the Temblor Range.

This is one of the least explored portions of the San Joaquin basin in which significant reserves might be discovered.