oa The Main Results of Investigation of the Crystalline Basement of the Tatarstan

The reservoirs of the basement have been studied, although the technology of studying the<br>Precambrian is yet to be improved.<br>Now, Tatarstan boasts of numerous wells drilled down to the crystalline basement, which indicate that<br>petroleum exploration at great depths does make sense. All previous evaluation of the crystalline<br>basement's prospects have been primarily based on the conventionally conducted drill-stem formation<br>tests (DSFT). However, oil and gas inflows from the crystalline basement of the Dnepr-Donetsk trough<br>showed that the sole use of DSFT results for evaluating the hydrocarbon potentials can be erroneous.<br>Moreover, the available drilling data from deep wells show that excessively high repressuring and the<br>use of loaded drilling mud can drive filtrate deeply into the reservoir. When the drilling rate is low, the<br>time interval between penetration and testing of prospective zones becomes quite long. If reservoir<br>pressure is equal to or is lower than hydrostatic, mud solution penetrates the reservoir and forms<br>seals, preventing formation fluid from flowing out during DSFT procedures.<br>Efficiency of hydrocarbon exploration within the Precambrian crystalline basement can be substantially<br>increased through the use of 1) a special drilling technology to minimize penetration of drilling mud<br>into the decompacted/fractured zones of the basement, 2) solutions that would minimize the influence<br>of drilling fluid on capacity and filtration properties of the reservoir, and 3) specific methods of inflow<br>stimulation and reservoir studies of the strata with various lithologic, petrographic and reservoir properties.<br>Seismic profiling and deep sounding revealed that the crystalline basement has a lamellar-and-scaly<br>structure. Main reflecting horizons have been found to occur below impermeable rocks at a depth of 5<br>to 7 km. Hydrocarbon fluids can be channeled into oil fields through the basement's fractures and<br>faults. Hydrocarbon relics from the fractured/brecciated zones indicate that the fluids could have been<br>driven from the lower horizons to the upper ones by the temperature field and the processes of<br>compression and decompression.<br>An obvious depth-related growth in amount of gas, a widening spectrum of methane's homologs,<br>greater amounts of methane's heavy ones, such as pentane and hexane, and the appearance of helium.