CHAPTER 10 PRACTICAL INSIGHTS IN DRILLING AN EXPLORATORY OIL AND|OR GAS WILL IN THE OVERTHRVST BELT

• Jurisdiction: United States

Overthrust Belt--Oil and Gas Legal and Land Issues
(Nov 1980)
CHAPTER 10
PRACTICAL INSIGHTS IN DRILLING AN EXPLORATORY OIL AND/OR GAS WILL IN THE OVERTHRVST BELT
Douglas L. White
Amoco Production Company
Salt Lake City, Utah

INTRODUCTION

The Overthrust is a geological area extending from Canada to Mexico. This area was created when the land masses of the west compressed to the east and actually slid over the top of the eastern land masses. Typical of this area is large displacement thrust faults extending 50 to 60 miles per fault. There are 5 major faults with numerous smaller faults.

The Utah-Wyoming area of the Overthrust, centered around Evanston, Wyoming, is composed of three of these major faults. To date, the only production is on the Absaroka Thrust Plate. Figure 3 shows a map of the area with the current producing fields. The discovery field was the Pineview Field in 1975. The discovery led to a massive exploration effort resulting in 19 new fields.

Figure 4 is a cross section of the Whitney Canyon Field. This is a typical crosssection of formations drilled in the overthrust. Shown are the discovery well and the offset well that was taken to the subthrust (below the Absaroka Thrust line). While the Thaynes formation was completed in the ACG #1 as a very good well, the primary horizons of interest in the field are the Madison and the Big Horn. The production from the Madison can reach as high as 50,000 MCFD and one well has been tested at over 15,000 MCFD in the Big Horn. The Big Horn is usually found below 15,000 ft and therefore is unregulated gas. In other fields in the overthrust, the Twin Creek and the Nugget can prove to be prolific hydrocarbon producers. These formations along with the Thaynes are "sweet" gas producers while formations below the Thaynes can contain up to 15% hydrogen sulfide gas. H_{2S is very toxic and great care must be taken in drilling and producing natural gas that contains H2}S.

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Exploring for oil and gas in the overthrust is a very time consuming and costly venture. The main exploration technique (other than drilling a well) is seismic operations. Noise signals are sent from the surface down through the various formations and the time it takes for the signals to return to the surface is recorded. An educated guess of the velocity of the sound waves through the formations is made and then with the help of an expensive computer program, the seismic surveys are computed and a seismologist makes an interpretation of the structure of the formation. From this, the geologist works on the prospect and attempts to draw cross sections of the horizons to see if there are any potential hydrocarbon traps. Traps are areas below the surface where hydrocarbons can collect and cannot migrate due to impermeable barriers.

Once the geologist determines there is a potential for commercial production, the drilling engineer estimates the cost of drilling and completing the prospect. This requires locating offset well records (if there are any nearby wells), determining a safe but economical casing and drilling program, knowing the current cost of equipment and supplies, and looking at topo maps to predict location construction costs based on access and terrain. From this information, a final well cost estimate is prepared and reviewed with management.

Once the well cost estimate is approved, the geologist and operations engineer compute economics based on "guesses" of initial production and reserves. Several cases are run using different production rates and reserves coupled with different probabilities of success. This will give management a better feel for what is required for a payout and potential return on investment.

Once these different groups have worked together and determined the prospect has potential, management must give their final approval. Before the management

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committee, the seismologist will justify his interpretation of probable structure. The geologist will explain the expected hydrocarbon trap and, if possible, compare it to other producing fields. The drilling engineer must be prepared to justify the proposed drilling program and the cost of the project. Then the operations...