Dolomitization Of The Lacustrine Carbonates Of The Toca Fm.(Kamba Field,Offshore Cabinda)And Quantitative Diagenetic Modeling

摘要

Hydrothermal dolomitization is one of the main diagenetic processes potentially affecting the petrophysi- cal characteristics of carbonate reservoir rocks. It’s effect on Barremian lacustrine carbonates of Upper and Lower Toca Formations has been investigated through an integrated diagenetic study and simulated through a quantitative diagenetic modeling(RTM TOUGHREACT software),in order to understand the behavior of dolomitizating fluids in the carbonate reservoir rock.This approach is also useful in carbonate marine contexts. Among the analyzed core samples,the best reservoir facies are represented by coarse Mollusk Coquina limestone and hydrothermal dolomite,which is the subject of this study. Two types of dolomite have been recognized.A finely crystalline,likely early diagenetic dolomite, shows low reservoir properties,and a coarsely crystalline,likely late diagenetic zebra dolomite,with very good reservoir properties. Dolomitisation is associated with strong silica precipitation in the Upper Toca Formation,pointed out by mineralogical and chemical analyses that also document,the presence of detrital palygorskite,typical of lacustrine environments under arid conditions and,in the Lower Toca Formation,of talc replacing ooids and indicating strong hydrothermal conditions. The integration of different investigation techniques allowed to hypothesize a mechanism of dolomiti- zation by hypersaline fluids probably mixed with modified hypersaline and fresh waters,interacting with crustal rocks,as pointed out by Strontium isotope analysis.High homogenization temperatures registered in fluid inclusions(100 to 140°C)could be due to crustal thinning,while the hypersaline fluids(20 to 25 NaCl eq.%)could be migrated from the Aptian Loeme Fm.salt or be originally present in the deep water lake sediments. The RTM TOUGHREACT software simulations pointed out that an important control on the results is exercised by the permeability field of the rocks.Dolomitization is directly proportional to the flow intensity,while temperature seems to exert less influence.From the sensitivity analysis water composition is very important for the process.