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The sediments which make up the reservoir rock for the MEX-GOL and SV fields are a sequence of sandstones of Lower Carboniferous age interbedded with shales and limestones.
The accumulations are stratigraphically, rather than structurally, trapped as the reservoir sands pinch out up-dip to shale.
Eleven gas and condensate bearing reservoir horizons have been identified to date, eight 'shallower' zones in the Lower Carboniferous (Visean) and three 'deeper' zones in the Lower Carboniferous (Tournasian). The Visean reservoirs, which begin at a depth of approximately 4,700 metres, currently form the main gas and condensate bearing horizons and reach a gross thickness of 800 - 1,000 metres within the field area.
To date, flow rates achieved in the Visean reservoirs have varied from 1.1 MMcf/d (31,000 m3/d) to 7.4 MMcf/d (209,000 m3/d). The Tournasian reservoirs begin at a depth of approximately 5,800 metres. These zones have a gross thickness of about 200 metres but have only been penetrated by five wells due to the depth limitations of the Soviet-era rigs previously used in these fields. Further, these zones are dry gas prone (i.e. have a low condensate content) and to date have only seen modest flow rates with the best achieved rate of approximately 2.9 MMcf/d (82,000 m3/d).
Owing to the limitations of the previous wells, significant uncertainties exist as to the degree of sand distribution and gas-water-contact depth associated with the Tournasian reservoirs. These reservoirs present a key source of potential future upside, not currently included in the 2P reserves estimate of 11.7 MMboe (ERC Equipoise, 2014). Further potential exists in the lower carboniferous zone, located at depths of around 6,000 - 6,500 metres.
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