S. J. MillsCanadian Occidental Petroleum Ltd. Calgary
In Dec. 18, 1991, Canadian Occidental Petroleum Ltd., on behalf of its wholly owned subsidiary, CanadianOxy Offshore International Ltd., and its joint venture partners, Pecten Yemen Co., Occidental Peninsula Inc., and Consolidated Contractors International SAL, announced that commerciality had been declared on three fields within the Masila Block in the Republic of Yemen.
The discovery and successful delineation of Sunah, Heijah, and Camaal fields-with estimated recoverable reserves of 235 million bbl-represents the climacteric of an exploration program which commenced in 1987 on a large tract of acreage located in the eastern part of what was then the Peoples Democratic Republic of Yemen. Drilling operations continue, and one further discovery has been announced recently. This article summarizes the exploration history of the Masila Block and discusses some of the results of the program to date.
PREVIOUS ACTIVITY
The Alif discovery made in 1984 by Hunt OH Co. in the Yemen Arab Republic and subsequent finds by the Soviet Technoexport, under contract to the government of South Yemen, prompted CanadianOxy and its original partner, Consolidated Contractors International, to acquire the 37,200 sq km Masila Block in March 1987 (Fig. 1).
The discoveries had demonstrated the presence of rift basins with a thick Jurassic fill in the Marib and Shabwa areas. Outcrop studies by Beydoun had described Jurassic outcrops on the western flank of the Mukalla high, suggesting that the Jurassic rifting extended farther to the east. 1
The section was considered highly prospective because of the presence of organic rich shales of the Madbi formation and potential reservoir sands of the Kohlan formation with the overlying salt of the Sabatyn providing a seal. Additional potential reservoirs were the Alif sands encountered by Hunt between the Madbi and Sabatyn formations and fractured Amran carbonates reported from the Technoexport discoveries. 2
At the time the Masila Block was acquired, little exploration work had been carried out in the area. Geological field mapping and airphoto studies, which were undertaken between 1937 and 1960 by Iraq Petroleum Co., and Technoexport, as part of its assistance program, had recorded aeromag and gravity over limited portions of the contract area between 1975 and 1979.
From 1978 to 1980 Agip SpA conducted Landsat-airphoto interpretation and a field mapping project along the coastal outcrop belt as part of its offshore exploration effort in the Sayhut Block. In 1981 Agip recorded 110 km of seismic on the coastal plain east of Mukalla.
To the north of the block in the Jeza syncline, Petroleo Brasileiro SA (Petrobras) shot a regional seismic program and drilled three unsuccessful test wells that penetrated an unpromising Cretaceous section. Petrobras relinquished the area in 1987.
GEOGRAPHY
The Masila Block topography is a result of the interplay of Tertiary rift tectonism and differential erosion. Three distinct geographic provinces can be recognized:
- The flat, narrow coastal plain is underlain by Gulf of Aden rift deposits of Oligo-Miocene age. Small fishing villages and the bustling old city and port of Mukalla hug the Gulf of Aden.
- The South Hadramaut arch, located 20 km north of the coast, marks the northern margin of the Gulf of Aden rift. Rising to a maximum height of 2,050 m, this 15-20 km wide uplift is splintered by countless normal faults and deeply incised by awesome canyons. Along the eastern margin of the block the canyon of Wadi Masila is the most spectacular as it downcuts more than 1,200 m before emptying onto the coastal plain.
- The interior plateau, north of the arch, lies between 600 and 1,400 m above sea level. Rainfall is less than 10 cm/year; vegetation is sparse, and human habitation is confined to the dry wadis, where hand-dug wells can reach limited perched water zones.
An estimated 1-2 km of gently northern dipping Tertiary strata have been stripped away, revealing a landscape dominated by resistive Paleocene limestones.
Local topography is determined by the interplay between Oligo-Miocene fault reactivation and this 250 m thick unit. In structurally low areas, monotonous plains and isolated mesas are found, but for the most part limestone fault scarps and incised canyons create an operationally difficult environment.
EXPLORATION PHILOSOPHY
From the outset it was realized that for the exploration effort to be effective it would not be sensible to concentrate on one small area only.
The challenge was to conduct a comprehensive evaluation effort that would highgrade the most prospective portions of this vast tract and allow a focused seismic effort to delineate drilling candidates.
The objective was to obtain enough information during the initial 3 year phase of the 6 year exploration term to decide whether or not to enter Phase II (which carried a well obligation). This task was made more difficult by formidable terrain difficulties over large parts of the block, which made seismic recording difficult and expensive.
Following a compilation and evaluation of the preexisting data, CanadianOxy initiated exploration in the Masila Block with a geological field survey in late 1987.
The purpose of the survey was a structural analysis to determine the tectonic evolution of the area and to corroborate the findings of earlier surveys concerning the source rock potential and general stratigraphy of the Jurassic and Cretaceous exposures. This was followed by a helicopter supported 2,000 station gravity survey conducted over the entire contract area between February and May 1988. The result of these surveys confirmed the extension of the Jurassic Shabwa graben into the southwestern part of the Masila Block and showed the Madbi source rock to be present along its southern flank.
A broad residual gravity low extending across the northern part of the contract area suggested the presence of another Jurassic graben. High gravity values without much variability in the central and southeastern portion of the concession are believed to indicate a thin sedimentary sequence over an area of shallow basement, roughly coincident with the South Hadramaut arch mapped by Beydoun from surface geology.1
SEISMIC CAMPAIGN
Based on the geological concepts and the results of the gravity survey, the seismic campaign started in September 1988. A Vibroseis crew was contracted from Geosource U.K. to acquire a reconnaissance program in the northwest area and a narrow strip along the northern concession boundary in the southwest area.
The relatively gentle terrain in the area of the program and the existing road network provided reasonably good access, and in 7 months 925 km of 60-fold data were acquired. Upholes were spaced 5 km apart at an average depth of 60 in for field statics calculations.
The initial interpretation results of the 1988-89 program were encouraging, identifying a number of seismic anomalies and appearing to confirm the gravity indications.
The second phase of the seismic program commenced in September 1989. Two seismic crews were contracted from Halliburton (formerly Geosource) and CGG.
The Halliburton crew operated in the northwest area, acquiring lines over previously identified leads, and was later moved to shoot a reconnaissance program in the east area along Wadi Masila and its tributaries. The crew completed the program in May 1990 after 1 046 km of 60-fold Vibroseis data had been acquired.
The CGG crew used a helicopter supported "pop shot" method to acquire 648 km of 60-fold dynamite data. The CGG survey consisted of three parts: a reconnaissance program in the southwest area; an in-fill program in the northwest area where no access to Vibroseis was available; and a few tie lines between the Vibroseis lines in the eastern part of the block.
The heliportable program was completed in April 1990. The average cost of acquiring the seismic data was $5,500/km for Vibroseis and $10,000/km for the heliportable data.
Generally, the data quality was found to be both terrain and surface cover dependent. Lines shot in the northwest area with its relatively gentle terrain were of good data quality, while the extreme rough terrain in the southwest resulted in considerable deterioration due to scattering by surface faults.
A comparison of data quality in the northwest area indicates that the best records are obtained when shooting over surfaces covered by the Eocene Jeza shale formation. Lines shot over the hard and weathered Umm Er Radhuma limestone or Rus anhydrites are of inferior quality due to seismic energy absorption and/or poor coupling with the vibrator pads.
Good seismic character match between the different types of data was achieved during processing. However, seismic time misties between the Vibroseis and the heliportable data were detected.
It was later concluded that the use of the first breaks method in calculating field statics for the heliportable data yielded different results from those obtained by the uphole method used for the Vibroseis lines. In order to minimize the time misties, control points at the intersection between the heliportable and the Vibroseis lines were established, and interpolated uphole field statics were used to adjust the heliportable data.
DATA INTERPRETATION
Migrated stacked sections were used for the seismic interpretation in the northwest area of the block. The exact geological identification of the seismic markers was uncertain due to the lack of control, but tenuous correlations were made with traded seismic data in the vicinity of the Jurassic outcrops to the southwest and to the Petrobras Al Furt well located 120 km to the northeast.
Although a basement event could not be readily identified on most of the data, a few lines in the northwest clearly showed a package of strong, deep reflectors, which was interpreted to represent the prerift Kohlan/Shuqra sequence overlying basement.
This prerift reflecting package appeared to be down-faulted to the west into an area where negative residual gravity values had been measured, and it was concluded that Jurassic rifting extended into the northwest part of the block.
In search of favorable hydrocarbon traps, seismic maps were generated for the most reliable markers in the northwest area. Several seismic structural closures were identified, representing potential hydrocarbon trapping locations within east-west trending Jurassic grabens.
DRILLING PROGRAM
Encouraged by these results and now joined by Pecten, the joint venture elected to embark on a two well drilling program.
The Sunah prospect in the extreme northwest corner of the contract area was selected for the first test (Fig. 2). Although not the largest closure recognized, it appeared to present the thickest Jurassic sedimentary section and was best located to receive hydrocarbons generated in and migrating from a thick Madbi section within adjacent half grabens.
Objectives were postulated to be Kohlan sandstone sitting on basement and sealed by Shuqra limestones; sand lenses within the Madbi shales, porous and/or fractured limestones of the Naifa formation; possible sandstones interbedded with shales within the Qishn formation; and, if present, an Alif sandstone equivalent.
The 1 Sunah commenced drilling in September of 1990 and reached a total depth in basement of 2,732 m on Dec. 25. A total of 4 m of oil-bearing Kohlan sand was found and 6 m of pay in the uppermost sand of the Barremian Qishn formation.
A cased-hole test of the Kohlan formation flowed 36 gravity oil to surface at a rate of 3,767 b/d. In addition, 30 gravity oil was recovered from the Qishn sand at a depth of about 1,700 m.
Tying the well with the seismic necessitated some changes to the original interpretation. While the basal prerift sedimentary package was correctly identified, some departures were encountered in the overlying stratigraphy.
Of note was the identification of an early Cretaceous platform carbonate unit that was named the Saar formation. A fortuitous discovery was the degree and quality of the Qishn sand development, an interval thought most likely to be carbonate and shale dominated. The revised stratigraphy of the northwestern Masila Block is shown in Fig. 3.
When testing of 1 Sunah was completed, the well was suspended and the rig was moved to explore the Heijah structure located 25 km south of Sunah. The 1 Heijah reached its total depth of 1,854 m towards the middle of March 1991. No Jurassic sediments were encountered over this prominent basement high, but oil was found in two sands within the Qishn formation.
Undoubtedly, the most significant well drilled to date was 2 Camaal located on the 26 sq km Camaal structure situated between the Sunah and Heijah discoveries.
This well encountered 29 m of net pay in a 94 m gross column within sands of the Qishn formation. Subsequent successful appraisal wells 3X Camaal, 4 Camaal, and 5 Camaal have confirmed the extent of this large pool.
Using electric submersible pumps, the Qishn reservoir has produced oil from individual zones at rates up to 4,875 b/d. The 29-33 gravity oil has little dissolved gas (20 scf/bbl) and is low in sulfur, although some horizons have indicated the presence of H2S in relatively small concentrations.
DEVELOPMENT PLAN
For each of the discovery structures an appraisal program-which included additional infill seismic, appraisal wells, and extended production tests-was carried out.
With the recent acceptance by the Yemen government of the development plan, the joint venture partners will pursue an accelerated development project, which anticipates first production during the last half of 1993.
It is estimated that some 19 wells will be required initially with up to 46 for full field development. The 1 Sunah Kohlan formation will be placed on production to evaluate reservoir performance and to estimate recoverable reserves.
The field facilities will comprise a gathering system, central processing unit, a 24 in. pipeline 160 km to the coast, and an onshore export terminal with 2 million st-tk bbl of storage and offshore loading.
The preliminary facilities and pump station design is for an annual production rate of 120,000 b/d of oil. Costs for the development wells, associated facilities, and pipeline are estimated at $500 million.
ACCELERATED EXPLORATION
Spurred by the early drilling success, the joint venture took on two additional drilling rigs and a service rig and embarked upon an accelerated drilling program.
A further 2,000 km of seismic were undertaken utilizing two heliportable crews and a Vibroseis crew. Such a level of activity was dictated by the need to thoroughly explore the remaining 27,500 sq km contract area (reduced by a 25% mandatory relinquishment in 1989) prior to the end of the exploration terms in March 1993.
To help achieve that aim, the southwestern and northeastern portions of the contract area were offered for farmout. Consequently, upon this initiative, Occidental Petroleum embarked upon an earning program in the northeastern area. At the same time it earned a working interest in the Masila production sharing area by acquiring a 10% interest from CCC and 8% from CanOxy.
Seismic acquisition continues to the east of the discovery area and continues to mature prospects for drilling. In late January CanadianOxy announced a discovery at Hemiar, located 18 km east of Sunah (OGJ, Feb. 3, p. 29).
Although the area to the east becomes operationally difficult, several large structural features have been recognized, and the presence of Jurassic outcrops in Wadi Masila gives encouragement that the key play ingredients will be present.
In addition to the Qishn and Kohlan targets, other potential reservoirs have been recognized during the drilling program. An objective of the ongoing seismic campaign is to define subtle traps and identify new play concepts in the western portion of the contract area. As described, the oil prone Shabwa graben extends into the southwest part of the Masila Block, and this classic Alif-Shabwa play merits further investigation.
The results of the activity to date on the Masila Block have been very encouraging, and CanadianOxy is optimistic about the ultimate potential. Recently, a 2 year extension to the exploration term has been granted, which will enable the aggressive exploration program to continue.
REFERENCES
1. Beydoun, Z.R., "Geology of the Arabian Peninsula: Eastern Aden Protectorate and Part of Dhufar," U.S. Geological Survey Prof. Paper 560-H, 1963.
2. Maycock, I.D., "Exploration and Development in Marib/Al Jawf Basin, Yemen Arab Republic," Houston Geological Society Bulletin, Vol. 31, No. 5, 1987, p. 8.
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