LOW PERMEABILITY GAS CHARGED BIOTURBATED RESERVOIRS

In the past, trace fossil research in hydrocarbon reservoir rocks was almost always confined to exploration geology, however, recent research shows that ichnology has significant applications in production geology. Permeability enhancement in bioturbated media has been recognized in five interrelated scenarios: 1) Surface-constrained textural heterogeneities; 2) Non-constrained textural heterogeneities; 3) Weakly defined textural heterogeneities; 4) Cryptic bioturbation; and 5) Diagenetic textural heterogeneities. In low permeable gas charged systems non-constrained textural heterogeneities and cryptic bioturbation are by far the most important.

Considering that burrow-associated enhanced permeability is evident in the rock record, it is surprising that little consideration is afforded it in the geological literature. Perhaps one reason is the matter of scale. The permeability of geological media is a bulk character. Thus, the three-dimensional arrangement of sediment heterogeneity must be understood if flow behavior is to be modeled or predicted, which are particularly difficult to accomplish due to the scale and complexity of biogenic sedimentary structures. Overlooking the potential impact of these structures can lead to inaccurate assessments of the flow characteristics of a sedimentary rock, and misidentification or non-recognition of permeability streaks in a hydrocarbon reservoir. Again, this is especially important in gas-prone reservoirs where slight variations in permeability can have dramatic effects on storativity, reserve estimates and deliverability.

A developing play is low permeability gas-prone reservoirs dominated by non-constrained textural heterogeneities. Such zones can be laterally extensive and continuous and in most cases are interbedded with (or are) the source rock. Therefore the gas does not have to migrate into the reservoir but will accumulate in any internal zones that show permeability and porosity enhancement such as burrow systems. Likewise, cryptically bioturbated sandstones represent comparably isotropic dual porosity systems where the entire rock can represent the flow unit. Such units therefore, can control much of the flow capacity of the reservoir.