A review of braided fluvial hydrocarbon reservoirs: the petroleum engineer’s perspective
Authors: Martin John H. 
Published: April 2008

Braided fluvial reservoirs form some of the world’s giant oilfields and are found in many petroleum provinces. At best, they have excellent characteristics, but where non-net intervals (shales and siltstones) form higher proportions, they are more difficult to appraise and develop. Geological characterization (lithological correlatability, vertical sequence and porosity/permeability variability) must be communicated effectively to the reservoir engineer, whose role is to formulate a development plan and predict the production profile for a field, and to monitor and optimise day-to-day field performance. He has at his disposal other techniques: cased hole neutron logging to monitor contact levels; well testing to determine reservoir flow capacity, boundary and layering effects; production logging to assess well inflow performance or injectivity; wireline pressure profiling, field pressure and production history analysis to define vertical permeability and to reveal reservoir compartmentalization and/or communication with aquifer or injected fluids.

Computer reservoir simulation modelling, now an essential tool for management of most major fields, has provided the impetus for advances in geological reservoir characterization. The question is: how can we adequately represent braided fluvial reservoirs in such models? At one end of the spectrum, sheet sands (although laterally variable) can be adequately mapped and modelled as ‘layered’ systems based on conventional approaches. At the other are multiple low-sinuosity channel sand reservoirs which, being uncorrelatable at well spacings typical during appraisal of offshore fields, may be suitable targets for stochastic or network — type approaches.

Although the behaviour of a hydrocarbon reservoir must be controlled at least in part by its sedimentological characteristics, the engineer’s view of a field may be somewhat different to that of the geologist. This is as true for braided fluvial reservoirs as for those of other environmental origin. The geologist and reservoir engineer must be fully aware of each others’ data, analytical methods and objectives in order to avoid wasted effort. In the future, despite greater computing power and better modelling software, specialist technical disciplines must not loose sight of the overall objectives of field development.

Small-scale geological variability affects the viability of enhanced recovery processes such as miscible flooding. However, at the development planning stage, more generalized parameter assessments may be sufficient. Reservoir characterization at the small scale, such as geostatistical outcrop analogue studies, should be balanced by more geological involvement in the planning and interpretation of well tests, which often provide the only dynamic data available during the appraisal and development planning of offshore fields. Greater cross-disciplinary understanding of reservoir characteristics could in turn ensure that an engineer would not develop an incorrect view of a braided fluvial reservoir, perhaps by correlating shaley intervals which just happen to be present at about the same location in the section.