Volcanic Weathering Crust And Its Reservoir Prediction Of Yingcheng Formation In Xujiaweizi Fault Depression, Songliao Basin

Volcanic hydrocarbon reservoir is a kind of important unconventional resource. Since it was firstly discovered in California’s SAN Joaquin basin of the United States in the end of the 19 th century, volcanic hydrocarbon reservoirs have been found in more than 100 countries area in succession. Researches on volcanic hydrocarbon reservoir is becoming on of the important domains in global exploration and development of oil and gas resources.Over the past 20 years, in Bohai bay, Songliao basin, Junggar basin and Santanghu basin, high-yielding volcanic hydrocarbon reservoirs have been found one after another, which indicates great exploration potentials of volcanic hydrocarbon reservoirs in our country.Compared with the organic volcanic rock, volcanic weathering crust is easier to form favorable reservoir and more productive reservoir, so the volcanic weathering crust plays a greatly significant role in volcanic oil and gas exploration. At present, scare research on volcanic weathering crust have been made, which are mainly concentrated in the upper Paleozoic volcanic hydrocarbon reservoirs in northwest China. Former researchers had studied pore genesis, pore distribution and development mode in volcanic weathering crust, which shows that favorable volcanic reservoir zones mainly developed in leached zone and disintegrating zone of volcanic weathering bodies. It was pointed out that volcanic weathering crust could form good reservoirs, which had important theory and application value as a special type of reservoir.According to the weathering age, the weathering crust’s thickness and structure, and its effects to the reservoir bed, etc. the Mesozoic volcanic weathering crust in the Songliao Basin is distinctly different from the Paleozoic volcanic weathering crust in Western China, but at present there is quite little research on volcanic weathering crust and its reservoir significance in Songliao basin. Therefore, this research is based on volcanic rock of Yingcheng group in Xujiaweizi fault depression region of Songliao basin, and its purpose is to explore identification mark of weathering crust of Mesozoic volcanic rock in Songliao basin, and to deeply and systematically study structure, development mode, distribution range and reservoir characteristic of volcanic weathering crust, which can lead to exploration and development of volcanic rock oil and gas in Eastern China.This study includes geological survey on field section of volcanic weathering crust, observation on drilling cores, microscopical observation on rock thin section and casting thin sections, whole rock analysis and SEM analysis of clay minerals. Combined with logging and 3D seismic weathering crust identification, forward seismic modeling and attribute analysis, etc, distribution range of volcanic rocks of Yingcheng group in Xujiaweizi fault depression region is established. Based on basin structure,eruptible stage and so on, the development model of volcanic weathering crust is built, and its reservoir distribution is predicted.This study has two innovative points :1. To clarify geological and geophysical characteristics of volcanic weathering crust of Yingcheng group in Xujiaweizi region of Songliao basin, to point out that the formation mechanism of weathering crust and its structure, to establish its identification mark, finally to set up the development mode of weathering crust of Yingcheng group in Xujiaweizi region.2. to refine reservoir genesis of weathering crust of Yingcheng group in Xujiaweizi region, to form geological and geophysical methods of reservoir prediction of volcanic weathering crust, to point out reservoir’s favorable area of weathering crust.Through this study, conclusions and cognition are as follows:1. Geological identification mark of volcanic weathering crust of Yingcheng group in Xujiaweizi fault depression region is built, structure of volcanic weathering crust is divided and its formation mechanism is summarized.(1)There are three classes of geological identification marks of volcanic weathering crust in 11 kinds: macro-characteristics: weathering clay, high angle cracking infilling clay zone, corrosion hole and amygdaloidal structure corrosion hole; micro- characteristics: carbonatation, laumontitization, chloritization, sericitization; clayization and microfracture; geochemical characteristics:weathering index(CIA) > 50.(2) The volcanic weathering crust in Xujiaweizi Fault Depression, Songliao Basin is divided into four layers from top to bottom: clay layer, leached zone, fracture zone and parent rock. Leached zone has a strong leaching alteration, developed corrosion holes and fractures, poor structure integrity of rock, and is the major oil and gas reservoir. Fracture zone has a strong leaching alteration, developed pores and fractures, semi-fractured rock, poor structure integrity of rock, and is the minor oil and gas reservoir.(3)Formation mechanism of volcanic weathering crust of Yingcheng group in Xujiaweizi region is summarized as follows: uplift and sag tectonic framework by strong fault depression contributed to the formation of volcanic rock weathering crust; Affected by Xu Zhong and Xudong strike-slip fault, volcanic formations in the region of developed faults obviously steepened, and suffered from intense weathering and denudation, so it formed volcanic weathering crust with larger thickness; upward migration of deep volcanic hydrothermal fluid along the fracture made the hydrothermal alteration affect the evolution of volcanic reservoir, and fracture development increased the thickness of weathering crust and effective reservoir.2.Geophysical identification of volcanic weathering crust of Yingcheng group is build.(1)Characteristics of the logging curve of volcanic weathering crust is summarized as follows: larger hole diameter, and it is usually more than 9-11cm; higher natural gamma ray value, and it is usually more than 120-150API; larger interval transit time value, and it is usually above 60-90μs/m; lesser density, and it is usually less than 2-2.5g× cm-3.(2)Characteristics of the image logging of volcanic weathering crust is summarized as follows:: the FMI image color of the upper weathering crust is bright yellow or dark yellow as same with the lower, in a nutshell, a lighter color, and the resistivity is high;but the FMI image color of weathering crust’s layers is brown or dark brown, in a nutshell,darker color, and the resistivity is low. When the weathering degree is weak, FMI image color of the weak weathering crust is dark red,and the resistivity is middle; When the moderate degree of weathering, FMI image color of the moderate weathering crust is red-brown, and the resistivity is low; When strong weathering degree, FMI image color of the strong weathering crust is dark color, and the resistivity is low.(3)The seismic response characteristics of the volcanic weathering crust are summarized as follows; strong reflection – continuous – low frequency same seismic event, the lower strata of the basin rim are terminated and the lower strata in the basin are coordinated. According to the seismic response characteristics of volcanic weathering crust, sensitive attributes are optimally selected-amplitude and frequency applied, and the maximum peak amplitude and the average instantaneous frequency attribute are extracted and in the seismic section of a length of weathering crust, and the distribution range of weathering crust is predicted. Percentage of accuracy of both attribute prediction is 81.43% and 72.86%, respectively.3. Reservoir characteristics of volcanic weathering crust are studied, its reservoir development model built and its reservoir distribution range predicted.(1)Corrosion holes and corrosion fractures by weathering are developed in a lot of weathering crust reservoirs. By semi-quantitative calculation of surface porosity, weathering is proved to effectively increase reservoir space, which improved the physical properties of reservoir. Volcanic rock with developed primary pores produce more secondary pores by weathering; Volcanic rock with not-well developed primary pores produce less secondary pores. Acid rock is more likely to be weathered than basic rock, which consists in the degree of crystallization of acid rock.(2)Two development modes of volcanic weathering crust are established in Xujiaweizi region: the paleo uplift model and the slope belt coexists with the sag model. Structure of the weathering crust in paleo uplift model is more integrated, including clay layer, leached zone, fracture zone and mother rock, usually forms good industrial gas reservoir. Structure of the weathering crust in the slope belt coexists with the sag model is not integrated, including only leached zone, fracture zone and mother rock, usually forms a good industrial gas reservoir or low-yielding gas reservoir.(3)Prediction principle of volcanic weathering crust is established: three classes of geological characteristics of volcanic weathering crust in 11 kinds, characteristics of logging curve, characteristics of seismic reflection and seismic attribute. Distribution range of volcanic weathering crust is predicted. Using data of the remaining 10 drilling well to verify the accuracy of the prediction, the results show that the absolute error and relative error are small, which indicates the accuracy of prediction. At last, favorable reservoir zones of volcanic weathering crust is predicted, reservoir tape Ⅰ is concentrated distribution in the middle of Xushen area, in the north and southwest of Dashen area; reservoir tape Ⅱ is concentrated distribution in the southeast of Xushen area, in the middle of Dashen area; reservoir tape Ⅲ is concentrated distribution around Xushen area, in the Songshen area and Shengshen area.