THE RELATIONSHIP BETWEEN AUTHIGENIC KAOLINITE, RESERVOIR QUALITY AND TRAP HISTORY IN WONNICH-I

A significant decrease in permeability is seen within the oil and water legs of the Wonnich-1 oil and gas discovery well in TP/8, despite an increase in porosity of 4-5 per cent by volume. The anomalous zone contains abundant authigenic kaolinite. The prompt integration of wireline log and core data allowed the presence of this kaolinite to be postulated at an early stage in the appraisal of the well. It also led to the proposal that the authigenic kaolinite was derived through the dissolution of detrital potassium feldspar and that its distribution was related to a palaeo-hydrocarbon contact. This hypothesis was used to design a tailored special core analysis and petrographic study including fluid inclusion, mercury injection and critical velocity analyses. The results confirmed the model and indicate that the bulk of the additional porosity within the anomalous zone is microporosity. In addition, the results show that where water is the flowing phase, the authigenic kaolinite becomes mobile at an interstitial velocity of approximately 4.4 jim/s. This equates to a water offtake rate of 5,700 BWPD, making kaolinite movement in the immediate vicinity of the well bore likely in any Wonnich oil development scenario. Pore throat sizes in the Wonnich reservoir appear to be sufficiently large that the kaolinite will pass through leading to an increase in permeability rather than a decrease. As a consequence, kaolinite movement is likely to lead to an increase in water production since it will occur preferentially in areas of water breakthrough.