| Compared with conventional reservoirs, shale gas has more occurrence ways, complex reservoir rock composition, changes in the larger abundance of organic matter type, strong heterogeneity, and diversity pore space, these characteristics determine the interpretation and evaluation of shale gas is difference with the regular oil and gas reservoirs, higher demand is required for its evaluation ideas and methods.The domestic successful experience of shale gas exploration and research in North American produced a huge inspiration for China, making shale gas as a kind of unconventional oil and gas resources to be a hot study area in recent years for China. Currently, studies of shale gas exploration is still in the initial stage, according to our complex geological conditions and tectonic setting, and there is a considerable difference compared with North America, to be successful in large-scale shale gas exploration is not easy.The study area of this paper is Jiaoshiba area in Sichuan Basin, the exploration of shale gas in this area started in 1950, and until a few years ago it was formally entered a substantive stage and in 2012, the well JY1 drilling oil and gas shows successful in upper Ordovician Wufeng Formation-the lower part of Silurian Longmaxi Formation, make a major breakthrough for Sinopec shale gas exploration work, then one after the other wells drilled oil and gas shows in the same object layers. The successful of shale gas exploration and development in Jiaoshiba areas provides a valuable experience for our shale gas production, and played a good role model for pioneering breakthroughs in shale gas exploration.This paper focus on the impact of gas containing which changes with the main factors of shale gas accumulation, the main research contents and methods are follows:1. After the preliminary research, understand the geological setting and geological characteristics of the shale reservoir in Jiaoshiba area. The thickness of shale formations in the target layer Longmaxi period (including Wufeng Formation)is between 80-118 m, distribution of porosity between 1.87%-7%, Ro is between 1.05 %-3.4%, TOC is between 0.86%-4.2%.2. The lower formation of Jiaoshiba is typical shale gas reservoirs, through analyzed the characteristics of typical well logs, Including statistics the well log values of different lithologies combination and the intersection of petrophysical parameters of different lithologic shale gas reservoirs of typical wlls, to analysis the petrophysical characteristics of different lithologic shale gas reservoirs.3. Single-well evaluation. As new drilling wells, the logging curve type of Well JY1, JY2, JY3 is more comprehensive, including gamma logs (GR), longitudinal wave (AC), shear (DTS), the resistivity curve class (RD, RS, etc.), the mineral content curves and other logging sequence curves. However, there is a lack of porosity log interpretation and other curves, which also needs to be single-well modeled and evaluated. For Longmaxi shale formation in Jiaoshiba area, mainly modeling and predictive Ro, TOC and porosity, through literature research to knowsingle-well evaluation methods of shale gas geology main factors which was commonly used, Combined with the real situation of Jiaoshiba area, choosing Ro-φ fitting to achieve the Ro forecasts; using Δ 1gR method to evaluate and predict TOC; using volume model method to calculate the porosity.4. etrophysical analysis. Through research rock physics modeling methods commonly used at home and abroad, and consider the real situation in Jiaoshiba area, establishing the shale gas volume model of Jiaoshiba area, then using Gassmann equations to carry out the rock physics simulation which changes with porosity; Brown-Korringa equation for TOC; design variable thickness curve for thickness variation simulation.5. AVO forward modeling. In the study of this project, using the original shear wave velocity and density curve which porosity and TOC corresponding as a benchmark, respectively reduced by 2%,1%, increased by 1%,2% and 3% to create six different porosity and TOC state. then simulation each one’s cruves. And then using AVO modeling to analyze AVO response characteristics which changes with porosity and TOC. Thickness variation modeling mainly through designed the target layer by a certain thickness, edit the original curve to simulate the density and shear wave velocity curves of different thicknesses, and then analyze the AVO response characteristics changes with thickness.Through the above analysis and research, get the following conclusions and understanding:1. Through Log variation analysis, GR and CNL become larger with depth increasing, SP and DEN becomes smaller with depth increasing, Resistivity, the overall trends of AC and DTS perform three sections, the upper section (section 3) and under section (section 1) is low resistivity, low AC and high DTS, middle (section 2) is higher resistivity, high AC and lower DTS.2. Statistical analysis of lithologic logs shows, GR, DEN, CNL can distinguish different lithologic and lithologic changes as monotonically decrease, while AC and DTS can basic distinguish carbonaceous shale and siliceous gray shale.3. Through the intersection analysis of petrophysical parameters of the target layer shale formations, including Vp & Vs, Vp & ρ, Vs & ρ, Zp & Zs, Zp & Vp/Vs, λρ&uρ, ρ & Vp/Vs, ρ & σ and K & μ and so on. Vp & ρ, Vs & ρ, ρ& Vp/Vs, ρ & σ four parameters can effectively identify and distinguish the different lithological shale formations, which explain these four parameters has good sensitivity for different lithologies shale formations, can clearly indicate the "three-stage" features of the target layer.4. Correlation analysis of the main geological and petrophysical parameters, the shear wave velocity and density of three small sections of the target layer, must has or better corrslation respectively, with TOC and porosity, and the shear wave velocity of seven lithology combination will change with TOC and porosity’s change. 5. According to the AVO forward modeling analysis of well JY1, the reflection coefficient of target layer’s top interface is negative, and negative decreases with the increasing of angle, P-G intersection points is in the second quadrant, the P value is Negative, the G value is positive, and has the same trend with the reflection coefficient. With the increasing of porosity, the P value decreases, the G value increases, the intersection point are in the Ⅱ quadrant, and the magnitude of G value changes larger than P value; with the increasing of TOC, the P value decreases, the G value increases, the intersection point conversion from the Ⅲ quadrant to the Ⅱ quadrant, and the magnitude of the P value changes greater than the G value; Along with the increasing of thickness, AVO response transformed from the IV quadrant, pass through the Ⅲ quadrant to the quadrant Ⅱ. |
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