The Masila Fields, Republic of Yemen

Abstract Masila Block 14, located in the Hadramawt Region in the east-central Republic of Yemen, is operated by Canadian Nexen Petroleum Yemen (a subsidiary of Nexen) on behalf of its partners, Occidental Peninsula and Consolidated Contractors International. Oil was first discovered in late 1990, with commerciality declared in late 1991. Oil production began in July 1993. There are now 16 known fields containing 56 pools. At year-end 2000, total proven ultimate recoverable oil reserves are 891 million bbl. Proven, probable, and possible reserve estimates are approaching 1.2 billion bbl of recoverable oil. The Masila fields are associated with the Upper Jurassic to Lower Cretaceous Say'un-Masila rift graben basin. Almost 90% of the oil reserves discovered are in the Lower Cretaceous upper Qishn sandstones, Qishn Formation, Tawilah Group. Oil also is found in seven other reservoirs consisting of Lower Cretaceous and Middle to Upper Jurassic elastics and carbonates as well as fractured granitic basement. This chapter focuses on the main oil-producing reservoirs, which are the informally named upper Qishn sandstones of the formal Qishn Clastics Member, Qishn Formation, Tawilah Group. The upper Qishn sandstones represent a transgressive sequence from braided river deposits into tide-influenced shorelines, overlain by subtidal and shelf deposits. The upper Qishn sandstone reservoirs have high porosity (15-28%) and high permeability ( 10 d). They are relatively homogeneous and continuous in the lower half of the formation and are more heterogeneous and discontinuous in the middle to upper sections. The uppermost marine sandstones are more homogeneous because they are texturally more mature. The major field accumulations are tilted, normal-fault block structures located over basement paleohighs and dependent on cross-fault juxtaposition against overlying Qishn Carbonates Member top seal. The carbonate-dominated pre-Qishn section, including the source rock, is not present on the paleohighs and is thickest in the basement lows. The main source rock is the Jurassic (Kimmeridgian) Madbi Shale, a Type II marine source that is mature in ''kitchens" adjacent to the structural highs. Secondary oil migration occurred upward along fault planes to the overlying traps. At year-end 2000, 2087 km (1304 mi) of two-dimensional seismic and four three-dimensional (3-D) seismic programs totaling 422 km 2(165 mi 2) have been acquired in the confines of the current Masila Block 14 boundary. Seismic acquisition in the Masila Block has been challenging because of the remote location, rugged topography, and rocky desert terrain. The land surface is incised by deep wadis, or canyons. Processing and interpretation problems are significant because of a low-velocity surface layer, scattered seismic energy, poor signal-to-noise ratio from numerous canyon walls, and "fault-shadow" velocity anomalies overlying many of the tilted fault-block culminations. The biggest production challenge is water handling. Much water is produced along with the oil because of a combination of medium-gravity (15°-33° API) moderate-viscosity oil, high reservoir permeability, and a strong regional aquifer. The upper Qishn oil is undersaturated in gas (average gas-oil ratio is 3-7 scf/bbl), requiring electric submersible pumps to provide sufficient artificial lift for the large volumes of produced fluid. At the end of December 2000, the annualized daily production rate collectively for all fields was 230,000 BOPD, with 725,000 BWPD and 6.8 mmcf solution gas/day. Cumulative oil production is more than 500 million bbl. Initial average well oil-production rates vary by producing zone but range from 1500 to 20,000 BOPD, with a few wells producing from more than one reservoir zone by minor zone commingling. In the early production years, oil and water produced in the fields were transported via pipeline to the central processing facility (CPF), where most fluid separation occurred. More recently, the majority of the separation of oil and water is being performed at individual fields using field-based hydrocyclones before transporting the "clean" oil to the CPF for final processing. Produced water is reinjected into the reservoirs. The clean oil is transported to the southern coast via a 140-km (85-mi)-long, 61-cm (24-in.) pipeline over a 106-km (66-mi) distance. Export oil is then loaded onto tankers via a single buoy mooring system located 3.2 km (2 mi) offshore east of the coastal village of Al Mukulla.

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