Bedding Corridors as Migration Pathways in Abu Dhabi Fields

Abstract

Hydrocarbon migration pathways control the distribution of oil and gas in Abu Dhabi sedimentary basins and therefore it is one of the most important and controversial problems in petroleum system pillars. The complexity of this problem appears in full of regards to the faults and fractures, as most of them are cemented, while the bedding planes forming corridors that are playing an important role in turn. Therefore, it is necessary to identify the direction, dominant form, and distance of oil migration. The role of bedding corridors in migration of hydrocarbon was not fully estimated. This paper introduces the bedding corridors as an important migration pathway in Abu Dhabi. They are sub-horizontal tabular bodies of bedding plane weak zones formed as shear swarms, which vertically divide the entire reservoir thickness and extend laterally as their layers. Bedding corridors are genetically related to shearing along the bedding surfaces and they contribute to the conductive systems in Abu Dhabi reservoirs. Bedding corridors can be best identified from horizontal borehole image logs, and as spikes in open-hole logs. An integrated study comprising logs, cores and outcrops are the main sources to delineate them. The significance of this approach is that once a diagnostic signature can be recognized in open-hole logs, in combination with lost circulation data, it can be used to allocate bedding corridors in other vertical wells with no image logs. Results indicate that the horizontal stress anisotropy is the key parameter to predict bedding corridors behavior under different stress regimes, as the Shmin and SHmax is usually very close to each other in most of Abu Dhabi reservoirs. Swapping stresses with depth is common in Abu Dhabi fields, where a strike-slip regime can open and dilate the shear stress along the bedding planes. Strike slip faulting regime, wich prevailed Abu Dhabi reservoirs is the main source of the stresses that distribute along the bedding planes, where most of the stresses are dissipating horizontally comprising the strike slip component. Bedding corridors can be considered as indicators of the reservoir stress history, where they form extraordinary clusters of a huge number of shear conductive corridors in a transpressional regime. In Abu Dhabi oil and gas reservoirs, bedding corridors networks help drain hydrocarbons. In carbonate formations, for example, it is quite common to observe a permeability contrast of about 500mD or more between the rock matrix and surrounding bedding corridors. In this paper, we differentiate between image-log porosity and density-neutron porosity, which is regarded as secondary porosity attributable to bedding corridors. We also introduce here a working hypothesis about their effect on hydrofraccing being part of geomechanical assessment. Geomechanically, these corridors can be used efficiently to increase production particularly in tight and unconventional reservoirs. The role of bedding corridors is particularly important in reservoirs having a tight matrix. It is recommended horizontal drilling to maximize production through those bedding corridors compared to vertical wells. Even in fraccing design, there is a need to highlight the importance of bedding planes in helping fracs to propagate.