Abstract

B-NC186 field is located in the Murzuq Basin, SW Libya. Fifteen wells have been drilled up to July 2009; seven of them consisting of a pilot hole prior to the drilling of the horizontal main hole.

The producing reservoir is the Upper Ordovician Mamuniyat Formation. The field is conformed by the oil accumulation in one of the glacial palaeovalleys filled by the Mamuniyat Formation which was eroded and capped by the major Silurian marine transgression and gently deformed during later orogenic phases. Main regional seal is Silurian age Tanezzuf shale. Tanezzuft basal Hot Shale member acts as the main source rock in the Murzuq Basin.

150km2 of 3D reprocessed seismic data were reinterpreted in 2008 in preparation of an aggressive drilling campaign. The main objectives of that reinterpretation were:

  1. Exhaustive QC of the existing data: seismic versions, well coordinates, KB elevations, markers, deviation surveys and VSP surveys

  2. Map in detail the fault network affecting our reservoir

  3. Enhance the position of the boundaries of Hawaz palaeohighs

  4. Perform a reliable depth conversion with the objective of identifying untapped upside potential

A detailed analysis of seismic attributes: Grid based, Variance Cube and Volume attributes (Dip, Azimuth and DipAzi combined), made possible the definition of the fault network affecting Mamuniyat reservoir.

Some interpretation principles were established in order to accurately define the limits of the Hawaz palaeohighs optimizing the location of producers nearby B-4 Hawaz palaeohigh.

A new NW-SE oil-bearing structural trend was discovered between B-4 and J Field palaeohighs. Sedimentologically this structural high constitutes a deep palaeovalley containing more than 1,000ft of clean Mamuniyat sandstone (well B-14H pilot hole). The OOIP of this southwest extension amounts to more than 100mmbo.

An unexpected low velocity anomaly was encountered after B-13H pilot hole well results. The VSPs acquired on the pilot holes of wells B-13H and B-14H revealed that wrong dynamic corrections were applied for Top Mamuniyat due to the thickness reduction of Tanezzuft shale along this trend. Consequently, the seismic velocity was overestimated and Top Mamuniyat seismic reflector was interpreted deeper than found by the VSP.

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