The Sedimentary Facies Research Of Jiufotang Formation In Zhangwu Fault Depression

The faulted basin group in the south of Songliao basin were formed in early Cretaceous. Located between Siberian Plate and North China Plate, the southern area of the basin was in their transition belt. The total area of the faulted basin is approximately2×004km2.Zhangwu Fault Depression is in the southern region of Songliao Basin and as a whole it is a NNE-trending, early Cretaceous sedimentary basin superposing upon the inner Mongolian axis in the Sino-Korean platform. It is represented by a east-faulting and west-lapping single-faulted half graben-like faulted depression, which is situated in the north of Daleng faulted depression, in the west of Zhangdong fault depression and in the north of Yaobao faulted depression, NS-trending with the area about90km2.There are Yixian Formation, Jiufotang Formation, Shahai Formation and Fuxin Formation from the bottom to the top in the Zhangwu fault depression, with the highest organic carbon content in the source rocks of Jiufotang Formation and the organic carbon content of most areas over1.0%. In late2010ZW2well was deployed and penetrated. Its normal test daily oil production was3.16m, and after cracking the interval of1219.7-1272.lm, the daily production was10m3. That’s how the Jiufotang Formation reservoir of Zhangwu fault depression was discovered and it was proved that the fault depression has good exploration potential. Therefore, the research of the sedimentary facies of Jiufotang Formation of Zhangwu fault depression can provide favorable geological foundation for the further exploration and development.Integrated with drilling, logging, coring and seismic data, based on fine stratigraphic classification and correlation, the seimic phase analysis has been done making use of3-dimensional seismic data; the lithologic composition, texture and structure in the area have been determined through the core observation, and the lithologic facies types have been determined according to the petrologic characteristics of Jiufotang Formation of Zhangwu fault depression; after using the results of core facies analysis to calibrate logging facies, single-well facies analysis has been done, all the wells have been analyzed and non-coring well logging facies have been done based on sample log and gas logging data. Because of the small area of the faulted depression basin, multidirectional sediment resources, polyphasic sedimentary superposition and other features, it is impossible to understand the sedimentary facies of the study area by the normal logging facies analysis methods, so in the process of this study, based on the petrologic data deprived from the coring, controlled on a macro level by the results of seismic phase analysis, applied on a micro level of log phase analysis to calibrate, integrated with all the data, it has been realized that the objective interval was deposited in subsea fan and fan-delta environment. According to the research results of coring phase, seismic phase and logging, each of the microfacies has been studied, and it has been pointed out that the midfan area is the favorable reservoir belt and the favorable facies belt distribution areas have been determined, which lays geological foundation for finding the favorable rolling development targets, deploying the evaluation wells and confirming the proved recoverable reserves. The followings are research achievements.1. Zhangwu fault depression, as a minor half graben-like faulted basin, is represented by frequent tectonic activities and complex sediment filling, with the seismic event often branching off, pinching out, change of phase and so on; according to the inner structure and external form analysis of the seismic sequence, the downlapping, onlapping and truncation boundaries have been regarded as seismic sequence interfaces, and integrated with seismic phase calibration and logging data, the stratigraphic classification and correlation project has been determined and the stable tracing and enclosing of the whole area has been realized; through analyzing the seismic facies reflection configuration of Zhangwu fault depression and the combined in-line and tie-line tracing, it has been concluded that the sediment source lay in the east and north, and a fan-delta body was deposited in the north while three subsea fans in the east. ZW2and ZW3wells which have produced commercial oil flow are located in the eastern source.2. Jiufotang Formation sandstones contain pebbles, is represented by gray, gray-green and dark gray sandstone. There are generally plant remains. Jiufotang Formation contains light gray, gray-green, dark gray, gray-black or black clays, locally with shelly remains in the clays. Structures observed in the clays are often block structure and grading bedding. According to the core facies analysis, Jiufotang Formation was deposited in subsea fan and lacustrine facies, with the former subdivided into fanhead, midfan and lower fan, and the latter subdivided into lake shore, shallow lake shore and half deep sea-deep sea subfacies. The midfan has been further divided into distributary channel, inter distributary channel and front sheet sand microfacies.3. Jiufotang Formation is composed of7subzones with the Ⅲ, Ⅳand Ⅴsubzones as the interest intervals. The overlying and underlying Ⅰ+Ⅱ and Ⅵ subzones were predominated regionally by a suite of extra-low amplitude GR and high electric resistance mudstones, which is the standard marker bed in the area. VSP horizon calibration and synthetic seismogram calibration to calibrate the seismic phase, which has been combined with stratigraphic classification and verified each other to improve the accuracy of stratigraphic classification and correlation.4. Salinity of the formation water in the objective intervals are low, the SP curve amplitude is an extra-low anomaly value and the SP curve is invalid or partly invalid, therefore, it is not ideal to use SP curve to classify sandstones and clays. Low-gamma clays are widespread in the study area, and especially large suites of clays in Ⅰ, Ⅱ and VI subzones and part of the clays in Ⅲ-Ⅴ subzones and argillaceous sandstones and sandy mudstones are also represented by low-gamma values. Affected by the low-gamma clays, the natural gamma curve is invalid or partly invalid, so it is not proper to use natural gamma curve to classify sandstone and mudstone. In order to identify and divide lithology accurately, multiple logging information such as SP, GR, electric resistance and AC has been integrated in the research, combined with seismic facies, core facies and planar combination rules of various sedimentary types in the study area, the log facies model has been established and the microfacies planar distribution maps of Ⅲ, Ⅳ andⅤ subzones of Jiufotang Formation have been compiled.