'Giant field' prospects exist offshore Oman
Hussain Ahmed Al lawati
Adil Rabia Jamin
Ministry of Oil and Gas, Oman
Mark Spencer Jones
PGS Geophysical AS
Dr. Bjorn Wygrala
The offshore area of Oman is dominated by the Sohar basin, which is a Creta-ceous-Tertiary deepwater basin confined between the Omani continental shelf and the Makran Accretionary Prism, with its associated thrust belt. In the Early Cretaceous the Sohar basin formed on the outboard edge of the Oman Mountains. During the Middle-Late Cretaceous, the basin appears to be in a deep marine setting.
A major unconformity separates the Cretaceous from the Tertiary. During the Early Tertiary shallow marine carbonates and fine clastics were deposited. Subduction recommenced during the Eocene on the northern side of the Gulf of Oman, in the Makran and Baluchistan. Transpressional and transtensional forces affected the Sohar basin, resulting in structuring, which shows extensional and compressional features.
Offshore blocks in the 2003 Oman Bidding Round cover 45,240 sq km.
During the Miocene, uplift and erosion of the Arabian Plate margin resulted in major sediment input into the Sohar basin. As a result of sedimentary loading, major growth faults with their associated toe thrusts became active. From the Miocene to the Present there was continuous sedimentary input into the basin.
Transpressional structures were created by the Oman Mountain uplift. Four main plays are thought to exist in the Batinah/Sohar basin:
- The Oligocene-Pliocene turbidites which drape over the underlying structures of the Eocene (prove- nance area to the northwest/northeast of block 18)
- The Palaeogene Carbonate play (primarily Jafnayn Formation reservoir) sourced by Eocene or Maastrichtian claystones and sealed by Eocene claystones. In nearby onshore Barka No. 1 (drilled by Amoco in 1985), this formation demonstrated porosities of up to 17%. Eocene Seeb Formation, above the Rusayl Formation, may be a potential reservoir, which in Barka No. 1 showed porosities of 15-17%. The Seeb would be sealed by the Diba Formation claystones
- The Oligocene Reef play, which is analogous to reefs known in onshore outcrops and inferred from seismic to occur on thrust anticline crests. Source potential is provided by Rusayl and Thaqab Formations, sealed by Oligocene marls
- A Maastrichtian sandstone play with a provenance area of Palaeozoic coarse clastics and crystalline basement, proven by the results of Batinah Marine No. B1 which encountered oil and gas shows. Source and seal is provided by Upper Cretaceous claystones interbedded with the reservoir.
The presence of an active petroleum system in the Sohar basin is substantiated by numerous direct hydrocarbon indicators and live oil seeps detected by Infoterra from satellite images.
Potential sandstone reservoirs of Upper Cretaceous Campanian to Maastrichtian and Tertiary age are present. The sands are deposited as marine turbidites and sub-marine channel sands. Potential shallow marine limestone reservoirs are also known from Tertiary outcrops, onshore. Reservoir-seal pairs exist in the offshore setting where the reservoirs are sealed by deep marine shales. Oligocene to Miocene marine turbidties are also apparent.
Large prograding foresets are evident within blocks 18 and 41 and indicate changes in sea-level/tectonics within the area. These also give indications of the down-dip "lowstand" equivalent leads in the form of basin floor fans and "back-stepping" sand traps.
Structures of various styles are well developed in this concession. Most relate to Tertiary gravity tectonics, induced by a combination of a gentle basin-ward slope, ductile substrate shale (perhaps salt), and a thick cover sequence of semi-brittle marine sediments. Superimposed on the gravity tectonics are signatures of wrench faulting.
Seismic examples over the toe-thrust system show deltaic foresets and evidence of gas clouds
Closures are tens to hundreds of square kilometers in area, and reservoirs may be stacked in growth-fault rollover anticlines and collapsed-crest listric fault blocks. Diapir flanks and flower structures provide additional structural closures. Stratigraphic pinch-outs are additionally evident in seismic sections.
Two wells within the region have good indications of the charge potential within the Sohar basin. Both wells had gas shows within the Turonian-Cenomanian, which is well known as a world class source interval. Additionally, the Batinah-B1 well had oil shows within the Maastrichtian dark marine shales (1,000 m thick). Source rocks are also identified in outcrop and within well Barka-1 from the Eocene Rusayl Formation. They represent Type II/III source rocks with total organic carbon values of 6-10%.
Direct hydrocarbon indicators, like this flatspot, occur across the region.
Burial modelling on these source rocks suggests that in the key prospect areas a Maastrichtian source would be at peak gas generation and an Eocene source would be mature for oil. Gas chimneys are apparent within the blocks, and oil seeps are associated with the growth faults and fold belt within the area.
Three blocks are available, ranging in sizes from 7,920 sq km (block 18A) to 13,520 sq km (block 18B) to 23,800 sq km (block 41), for a total offshore area of 45,240 sq km. The main elements of the exploration and sharing agreement (EPSA) are an initial exploration term with one/two possible extensions, a 30-year production agreement in case of commercial discovery, which could be extended for an additional 10 years. Companies are exempt from paying taxes and import duties. A number of the terms within the EPSA are negotiable, making Oman a flexible and competitive environment to invest.
Wells BMA-1 and BMB-1, drilled on the shelf, give only an indication of the regional prospectivity down-dip in the Sohar basin. Both wells contain gas and oil shows. In 2002 PGS acquired 5,839 line-km of seismic and gravity data.
The Sohar basin is under-explored, with only two wells from 1968/72 (one drilled off structure), barely giving an indication of the regional prospectivity. The area consists of large structural closures that have "giant field" hydrocarbon potential. The key is utilizing the available seismic with its direct hydrocarbon indicators and amplitude anomalies to unlock the potential reserves. The Ministry of Oil and Gas announced the Gulf of Oman License Round 2003 in January, and it will close on July 15. Details of the bidding procedures and the exploration and production sharing agreement can be obtained from PGS Geophysical AS.