The Study On The Characteristics Of Gas/water Distribution Of The Fourth Member Of Xujiahe Formation In Xinchang Gas-field

Xinchang gas-field is located in Deyang city and in the Xiaoquan-Fenggutectonic belt, west Sichuan Depression. Its Xujiahe Formation is characterized bygeneral gas-bearing and large resource.However it’s restrained by ultradeep,overpressure, super-tight, strong heterogeneity, above which lead great difficulties toexploration and development. The productive capacities for the second and the fourthmember of Xujiahe Formation gas reservoirs vary and multiple layers have waterproduction with different yields.Therefore, mainly based on well-logginginterpretation, combined with mud-logging and testing data, etc, we have studied onthe characteristics of gas/water distribution in the fourth member of XujiaheFormation （T₃x⁴）, furthermore, to unravel its controlling factors in Xingchanggas-field and finally to predict present distribution status.It’s characterized with coveal gas and water in the same layer in the uppersection of T₃x⁴, while the lower section has larger gas saturation than the upper. Thewestern part of our studied area is situated in the high point of the structure, with agood gas-bearing status in each sand layers in T₃x⁴, while for the middle part,gas/water distribution situation varies with different sand layers: gas-bearing layersare mainly in TX₄²、TX₄⁹and TX₄¹⁰and water is widespread in TX₄³and TX₄⁴. As forthe east part, coeval gas and water is quite common in each sand layers of the uppersection of T₃x⁴and gas/water distribution was under the control of faults.By comprehensive analysis of the relationship between gas/water distribution inT₃x⁴and source, seal, reservoir, structure etc, it indicates that there is much larger watersaturation and smaller gas saturation in the higher section of T₃x⁴than the lower section,with such reasons as follow:（1） there was relative worse source for the upper section ofT₃x⁴than the lower, leading to insufficient gas charging, and thus the water was notenough expelled by gas; and （2） bad seal condition, resulting gas diffusion and escape,which could not migrate into reservoir to displace water. Although the reservoircondition of the lower section of T₃x⁴was not as good as the upper, it would besignificantly improved by the fracture. As for a certain sand layer of T₃x⁴, there ismuch larger gas saturation and smaller water saturation in the structurally higher partthan the lower part, indicating of a structure control on the gas/water distribution. Thus, we suggest that the controlling factors of gas/water distribution in T₃x⁴include theconditions of source, seal, reservoir and structure.We have predicted the gas/water distribution characteristics of T₃x⁴in plane: asfor upper section of T₃x⁴, there is a higher gas saturation in western and middle partsof Xinchang gas-field, while it is characterized by higher water saturation in easternpart near fault. TX₄³and TX₄⁴have unified gas/water contacts respectively.Thegas-bearing status in lower section of T₃x⁴is totally better than the upper. We suggestthat Tx₄⁹is the best target for gas exploration in Xinchang gas-field, in particular thesand body with fractures. Subsequent exploration location can be extrapolated,hundreds of meters lower than XC9well which is the lowest wellsite in this study.