New Methods for Quantitative Reservoir Quality Prediction in Sandstones

Abstract

Abstract The efficiency of a sandstone reservoir is a function of the initial depositional parameters (grain-size, sorting and composition) and of the post-depositional evolution during burial. The depositional parameters are closely linked to the sedimentary environment and the post-depositional evolution depends on the paleo-dynamic geological setting that conditions both the burial rates and the temperature history. The reservoir quality of sandstone was appraised in the past mostly in a qualitative way either by using close-by cored wells information or by taking into account the log porosity from adjacent wells. In the recent years more quantitative approaches to predictions were developed. The reservoir quality is currently appraised in Eni E&P with two methodologies. The first is proprietary and is based on the Reservoir Efficiency Index© (e-rei), a quantitative indicator of the quality of a clastic reservoir calculated from thin sections on the base of the quantitative petrographic data. A dedicated geostatistical tool (Softkrig) was developed in order to map the e-e-rei values on the base of TWT, depth or temperature driver maps.. The second methodology is based on the use of a reservoir quality modelling software (Touchstone™), developed in the frame of an Industrial Consortium and able to predict the mechanical compaction, the quartz cement precipitation and the illite formation and thus to model porosity and permeability. The methodologies are complementary and have been applied on Paleozoic African reservoirs. The main factor, reducing reservoir quality, was identified in the cementation by quartz, while the preserving mechanism, increasing reservoir quality, was recognized in the presence of early diagenetic clay coatings. The case-study shows the pros and cons of applying the two methodologies. In particular the e-e-rei mapping is recommended for the play fairway analysis of reservoir quality, while Touchstone is more suitable for the sensitivity analysis and for the determination of the porosity/permeability values on a given prospect. The two methodologies are limited one by the fact that is deterministic, the other by its ability to model only some of the diagenetic events that take place during burial. In order to overcome these problems, transport-reaction models may be used. A first attempt to apply these models, already used in carbonate diagenesis and reservoir quality prediction, to sandstones showed that there are way forward in applying this methodology. Introduction The Reservoir Quality Quantitative Prediction (RQQP) of a sandstone reservoir is a fundamental activity aimed at the delivery of maps that may be used for the evaluation at level of play/leads/prospects and, more in general, for the ranking of the different zones of an explorative area/concession. This provides a significant contribution to the explorative activity from basin to prospect scale. The reservoir quality of siliciclastic sediment, that will become sandstone, is basically determined by:the vertical (thickness) and horizontal (areal) dimension of the sand body; this is directly linked to the energy of the environment of deposition and influences the presence and amount of sand deposited in each point of the basin;the depositional initial petrographic characteristics (such as grain-size, sorting and the presence of detrital clays in the pore space) and the original composition of the sediment in terms of type of grains (quartz, feldspars, lithic grains…); both features are determined by the energy of deposition and, for composition, by the source-area of the sediments;