Depositional Features of the Middle Jurassic Formation of Field N and Their Influence on Optimal Drilling Schedule

Middle Jurassic formation of Field N presented by 4 hydrodinamically connected layers (J5–6, J4, J3 and J2) contains 42 % of field STOIIP. J2–6 formation is characterized as gas-oil-condensate massive lythologically and tectonically screened accumulation with gas cap (J2, J3 layers) and bottom water (J5–6 layer), oil is predominantly contained in J3 and J4 layers. There is high risk of early gas coning from gasbearing layers to oil producing wells indicated from production test results which can significantly decrease the life of well. For selection of more optimal drilling schedule it is necessary to take onto account resik of early gas coning and determine distinctive features within gas-saturated zone that can reduce it. Presence of thick shale barrier between J2 and J3 layers with range of thickness from 0 to 30 m is recognized as beginning of transgression cycle, and in case of gas cap containing only on J2 layer this barrier with thickness more than 5 m can extensively prevent early gas coning into oil producing wells. Integration of geological information represented by constructed probability map and petrophysical information represented by kh map allows more precise determination of optimal drilling schedule.