Tar mats and residual oil distribution in a giant oil field offshore Abu Dhabi

This paper describes how geochemical data (Rock Eval analysis, SARA composition) combined with wireline log interpretation allows for the recognition of the distribution and continuity of bitumens in a main reservoir of an offshore giant field in Abu Dhabi. The integration of new geochemical data with data and field information provided by the oil company ZADCO allows for the recognition of two types of bitumen rich levels in the main reservoir of the field: a) one corresponding to bitumen rich main reservoir intervals associated with high resistivity and high oil saturation, these intervals can be called “tar mats”, b) the other corresponding to low oil saturated intervals, and can be classified as “heavy residual oil”.In terms of lateral and vertical distribution, the tar mats are found at the crestal area of the Present-day structure and are located at the base of the reservoir unit above a tight limestone which plays a role of being a vertical permeability barrier. The tar mats seem to be independent of the Present-day OWC and are not related to biodegradation processes. The heavy residual oil is mainly located in the Northeast and the Southeast parts of the field and close to the OWC but it is also present all around the field except 1) in the west, in the area of the spill point and 2) in the Northwest area where direct contact between mobile oil and water is detected.Study of the structural evolution demonstrates that a tilting of the field began at Dammam age time (Eocene). The tilting of the structure led to a reduction of the structural closure in the West followed by the leakage of part of the originally trapped oil.Numerical modeling of such a geological scenario leads to a distribution of fluids (water, movable oil and residual oil) very close to the one observed at Present-day time in the field. This modeling allows a prediction of the extension and distribution of the residual heavy oil within the studied reservoir and can be used to better define an optimal production scheme. The effects of the residual heavy oils on the fluid circulation are difficult to evaluate, depending on the distribution of the bitumen in the pore network.     

储层中的沥青沉淀带及其对油气勘探的意义

在世界很多油气田的储集层中,都发现了天然形成的沥青沉淀带,说明储层中沥青沉淀带的存在是一个相当普遍的地质现象。前人的研究多集中在沥青沉淀的机理与成因判识、沥青的分布特征及其对储层物性的影响等方面,很少涉及沥青沉淀带对油气分布的影响,即使涉及的少数研究成果也都是从单一油田来进行分析,还未曾对其共性进行总结,此外,勘探家对钻探过程中出现的大量储层沥青重视不够,从而使得一些与沥青沉淀带有关的隐蔽型油气藏未能及时发现。根据前人的研究成果和实例,总结了3种不同成因储层沥青沉淀带的可能位置和储层沥青沉淀带的形成对油气分布的控制,指出沥青的沉淀对于油气藏不仅只有破坏作用,也有可能起到封堵作用而形成一种非常规类型的油气藏,关于这一点在勘探中应引起足够的重视。