Diagenetic Modelling In The Middle Jurassic Brent Sand Of The Northern North Sea
Authors: Hancock Nigel J. 
Published: December 2007

The permeability of the Middle Jurassic Brent Sand can become seriously restricted by the growth of authigenic clay minerals in the pore space. From examination of numerous wells in several northern Viking Graben oilfields, a model has been developed for clay mineral diagenesis in the sands. The following phases are recognised:growth of kaolinite in fresh waters introduced at the end of the Jurassic;precipitation of illite and illitisation of kaolinite under alkaline conditions in the Tertiary, except where the sands were protected by prior hydrocarbon accumulation;flushing away of alkaline fluids by fresher water.

This diagenetic model has diverse applications. In the first place, it frequently helps to rationalise porosity and permeability data. It can be extrapolated to predict nonproductive horizons, locations, or perhaps even structures. In the second place, the authigenic clay mineral profile can often enable recognition of the position and nature of oil/water contacts. In reservoirs which have undergone late stage tilting or leakage, it is sometimes possible to identify 'fossil' oil/water contacts. Hence the degree of tilting can perhaps be assessed.

The results in such a case may have important repercussions for a production programme. In particular, a simple case may be envisaged where, in addition to permeable oil reservoir, two significant zones may be distinguished. One zone contains oil but has very poor permeability: it should perhaps be excluded from reserve calculations. The other zone contains water, and has a good permeability: it may have relevance in the design of a water injection programme.

INTRODUCTION

A wide variety of diagenetic processes can take place in sandstones, involving the dissolution and precipitation of different minerals; most of these have an adverse effect on reservoir quality. Differences in amount and type of diagenesis can change the permeability of a single primary depositional facies of reservoir sand by as much as 1000 times. For this reason, a good understanding of diagenesis is of great importance at both exploration and field development phases of petroleum technology.

The earlier phases of research into diagenesis involve the identification and description of diagenetic problems: such work is in the nature of a post-mortem examination, carried out in hind-sight after wells have been drilled. There are innumerable examples of this type of research in the literature.

The science of diagenesis only begins to yield its proper value when it reaches the predictive stage: when the diagenesis at a new location ceases to be a mystery prior to drilling, and when variations in diagenesis over the area of a field are fully understood so that they can be incorporated into the development programme. In order to extrapolate this sort of interpretation from scanty well control, it is necessary to build and then test theoretical models for the types of diagenesis encountered in different situations. This paper describes the development of one such model, and demonstrates some of its diverse applications in problem-solving.