Petrology of Upper Silurian reservoir rocks from the Kudirka Atoll, Lithuania

The present report deals with a petrographical and petrophysical investigation and evaluation of rock samples and well logs from the Silurian Kudirka reef structure. It intends to answer folloving questions: (1) What diagenetic processes have occured within the different reef rock types from the time of deposition untill present day? (2) Have the diagenetic events influenced the reservoir quality (#phi# and K) of the different reef rocks in a different way? (3) If a correlation exsist between reservoir quality and reef rock type, is it then possible to make a well-to-well correlation of porosity, based on wireline log interpretation, and rock type? Irrespective of the site in the entire Kudirka reef complex, and irrespective of the rock type (rock texture), the reef carbonates have all been subjected to the same sequence of diagenetic events: Compaction/dewatering of lime mud #-># Early lithification #-># Formation of small fractures #-># Early leaching #-># First step of precipitation of intergranular/intragranular sparry calcite cement #-># Second step of precipitation of intergranular/intragranular sparry calcite cement #-># Recrystallization, a neomorphic alteration #-># Chemical compaction and fracturing #-># Precipitation of dolomite, pyrite and silica #-># Late leaching #->#? Oil emplacement. The first five events are assumed to have taken place in the marine or syngenetic stage, whereas the remaining events have taken place in the shallow burial or epigenetic stage to the intermediate burial stage. The present reservoir quality of the reff rocks is primary linked to two of these events: the late diagenetic leaching and the precipitation of sparry calcite cement, that have respectively improved and lowered the quality. The diagenetic transformation of the Kudirka reef rocks has ruined the expected relationships between reservoir quality and reef facies (rock texture): the proximal fore-reef facies being of good quality and the lagoonal facies of relatively very poor quality. Because of the diagenetic alteration no correlation is seen either in the Kudirka reef complex between the reef rock types and the reservoir quality (porosity) based on the wireline log interpretations. An examination of the spatial distribution in the reef complex of rock types (the reef architecture itself) must thus be based exclusively on macroscopic core descriptions. A well-to well correlation of porosity, based on log interpretation, with a subsequent construction of iso-porosity maps, seems very difficult to make for the Kudirka reef complex, because the porosity vary considerable even at very short distances. However, it is important to emphasize that the wireline log analyses are rather problematic, as both the porosity and the saturation interpretation are vitiated by uncertainties related to the limited resolution of the logging tools used, and furthermore, the use of relative high mud weights give rise to invation problems that complicate log interpretation