| The breakthrough which were occurred mainly in volcanic and conglomeratic reservoirs demonstrated the great potential for deep gas exploration in the Xujiaweizi rift depression. With the exploration speeds up, more and more geological problems should be resolved rapidly. Some of problems about the distribution of source rocks, the potential hydrocarbon generative of source rock, the genetic type of natural gases, the contribution ratio of different source rocks to mixed natural gases reservoir and Timing for gas charge in reservoirs have studied in this paper. Not only the advanced geochemical experimental techniques have been used in our studying work ,but also we developed some new techniques and methods, for example the adsorbed gas preparation technique, the enriching method of light hydrocarbon in natural gases and experimental technique of natural gas contribution ratio of four layers source rocks. The last one was awarded national invention patent. Being used the new techniques and combining geology analysis, some new conclusions will play important roles in exploration in this area.The spatial distribution and hydrocarbon generation potential of source rocks determine the resource potential and exploration strategy. Based on the data of drilling and deposition phase the thickness and spatial distribution of source rocks has been described. The dark mudstones and coal beds in the Shahezi Formation occur in much wider geographic area, with dark mudstones ranging in thickness from rift depression 30m to 350 m and locally up to1000 m. the dark mudstones in Huoshiling Formation distributed mainly in middle of the rift depression, and its thickness is less than Shahezi Formation. The dark mudstones in Yingchengzi and Denglouku Formation distributed locally and its thickness is much thinner than in Shahezi and Hulshiling Formation. The geochemistry analysis data (Rock-Eval, TOC and solvent extraction yields) of source rocks indicate excellent gas generative potentials in Shahezi Formation, good to excellent gas generative potentials to Huoshiling and Yingcheng Formation and poor to good gas generative potentials to Denglouku and Quantou Formation. Maceral identified and hydrogen index values of the studied rocks indicate most gas-prone type III organic matter, but the organic source input in the some potential hydrocarbon source rocks from the Shahezi Formation may initially contain some aquatic organic matter. The high temperature and high pressure simulation experiment has been used to study the process of hydrocarbon generation in source rock in Shahezi Formation and the process of gas generation by oil cracking. Both the models of hydrocarbon generation of source rock and oil and the equation about relationship between methane carbon isotope and source rock maturity (Ro) have established.The characteristic of composition and carbon isotope and hydrogen isotope of natural gases in Xujiaweizi rift depression have been studied. Also the genetic type of natural gases in this area has been classified. The gases are generally dry (i.e. with little C2+ hydrocarbons), However, there are wells that produce CO2 rich gases (e.g. Xushen No.9 well). Theδ13C values of gases are generally heavier, and most of gases have positive or negative carbon isotope series, few of them show reversal carbon isotope series. The complication distribution of isotope series suggested that the natural gases in the reservoir may be either mixing of gases generated at different source rocks or high thermal maturity source rocks or contribution of abiogenic gases. Using the quantitative calculation technique of the contribution ratio of different source rocks to mixed gases, Calculated the contribution ratio of four layer source rocks to natural gas reservoir in Xujiaweizi rift depression. The result indicates that most of the gases in the Xujiaweizi rift depression were derived from source rocks in Shahezi Formation. But the contribution ratios of source rocks show clearly dependence on spatial distribution of different source rocks. For example, the contribution ratio of source rocks in Shahezi Formation to gas reservoir in its source rocks developing area is can be up to 99%. And in the palaeo-central uplift, which developing source rocks in C-P, the contribution ratio of C-P source rocks to gas reservoir can be up to 85%. Similarly, the same regularity is coincident to source rocks in Yingcheng Formation. So, it is can be concluded that gases in Xujaweizi rift depression should be migrate vertically to reservoirs. The distribution of gases reservoir are controlled by spatial distribution of source rock. The distribution areas of principal source rocks are also the most favorable exploration zone for deep natural gases.Timing for gas charge in reservoirs have been studied by three techniques, including the hydrocarbon generation and expulsion of source rocks, carbon isotope fractionation during natural gas generation from source rocks and fluid inclusions analysis. The 1-D basin modeling also be used to define the geologic age of gases charge in reservoirs. The results suggest the timing of the gas charge was around 110Ma~90Ma in Shengping area, 82Ma~98Ma and 75Ma~63Ma in Changde area, 100Ma~90Ma and 78Ma~68Ma in Xingcheng area. This is slightly latter than or at the same time as the main period of gas generation from main source rocks.Finally, in Xudong and Anda area there are thicker dark source rocks distributed, where are predicted to be favorable exploration zone for deep natural gases in Xujiaweizi rift depression. Yingshan-Shuangchun rift depression, Gulong rift depression and Lingdian-Changjiaweizi rift depression also have gases generation conditions, will be next favorable exploration zone in the future.
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