The study on the contributing factors of phase state of gas condensate: examples from Tarim Basin

The gas condensate is a particular type of hydrocarbon system. Two methods are presented here to study the origin of gas condensate. Method 1 is based on the thermal simulation of two typical source rocks, humuic organic material and sapropelic material. We can get the compositions of generated product, and calculate the mole percentages of hydrocarbon. The Ro% values come from the residue generated during the thermal simulation. After these data are put into the software package for the PR equation, a series of phase diagrams of the generated product can be exported, which shows the PVT phase parameters at different thermal evolution stages, corresponding to different Ro%. The model of gas condensate formation then could be built. It is proved that gas condensate generated from terrestrial, humic source rocks with thermal maturities equivalent to vitrinite reflectance Ro% values between 0.5% and 1.9%, and from marine, sapropelic source rock with thermal maturities ranging from 1.5% to 1.9%. Method 2 is based on the analysis of the gas condensate sample from the reservoir. The delta(13)C(1) value of sample, determined by sample analysis, is used to distinguish the type of the source rock and its maturities by the formula, the conversion between delta(13)C(1) of hydrocarbon system to Ro%. So we can get the Ro% value of the gas sample and figure out its depth (D), temperature (T) and pressure (P), and then get a phase diagram of the generated product according to our model in Method 1. At the same time a present phase diagram of the hydrocarbon systems generated from source rocks can be calculated by PVT analysis. The difference between the two phase diagrams can be used to explain the formation mechanism of the gas condensate reservoir. The condensate gas reservoirs can be classified into three primary types according to their formation mechanism. (C) 1998 Elsevier Science Ltd. All rights reserved.