Study On Geophysical Method Of Sweet Prediction Of Shale Oil

The evolution of the oil industry will undergo three stages:the breakthrough of conventional oil and gas, the conventional oil and gas and unconventional oil and gas exists at the same time, and unconventional resources dominants. In 1934,McCollough put forward the theory of "trap", which marks the formation of traditional petroleum geology. The "continuous accumulation" theory proposed by Schmoker offers a scientific basis for the development of unconventional resources. Unconventional resources refers to those that can not get industrial production with traditional technology and needs new technology to improve the permeability and viscosity of the reservoir in order to realize its commercial value. It mainly includes the tight oil, shale oil and gas, coal-bed methane, fractured carbonate oil and gas, volcano rock oil and gas, oil or gas in metamorphic rocks, heavy oil, bitumen, oil shale, natural gas hydrate, etc. According to the organogenic theory, there will be a considerable number of oil or gas stranded in the source rockCompared with the conventional resources, shale oil has two essential difference: One is the oil or gas not controlled by obvious trap, the other is whether a single well has natural and industrial production. Although unconventional resources such as shale oil generally with low porosity and low permeability, but those with relatively higher porosity and permeability are favorable for production. As a result, the understanding and forecast of the "sweet spot" of the shale oil can effectively accelerate the use of unconventional resources in the early stage.At present, we have no successful experiences in make use of shale oil and its exploration and evaluation methods can only learn from the foreign countries. With the shale oil exploration data and core data from the key well in Ji yang depression and combining with experiences from domestic or abroad, we summarized the geological elements of the "sweet spot" as the following:(1) The organic matter maturity should between 0.7%～2.0% and organic matter content more than 2%. The content of the free hydrocarbons in shale is the key to its commercial value. The research results in Ji yang depression and other oil and gas-containing basins showed that within a certain range, free hydrocarbon increases with the increasing of the organic carbon content, while maturity can influence the colloid and asphaltene content in the free hydrocarbon.(2) The brittle mineral content is higher than 40% while the clay mineral content is less than 30%. One key to the successful development of unconventional shale oil is the breakthrough of horizontal fracturing technology. A certain amount of brittle minerals can guarantee a large number of connected fracture systems formed in the process of fracturing. The practice in Ji yang depression shows that a certain content of the carbonate rocks is the key to a high yield.(3) The existence of abnormal high pressure. At present, there are less index to evaluate the preservation condition of unconventional resources, but the existence of abnormal high pressure is an important symbol of the enrichment of shale oil. The production from well FY1 in Ji yang depression sharply decreases with pressure reduction also shows that pressure is an important condition for shale oil output.(4) Lower oil viscosity. Due to the low porosity and permeability of the shale, lower viscosity crude oil flow more easily. The production data comparison between well G7, well F119, well N52, well w76 shows that the production capacity decrease with the increasing of viscosity.(5) The size of the volume of shale. A certain amount of shale can ensure the operation of industrialization.Geophysical technology is an important means to the evaluation of shale oil resources.In this paper, the main work carried out are as follows:(1) Summarized and made a comparison of the present TOC forecasting method and their application condition. Established a TOC forecasting model which is applicable for the Ji yang depression by using conventional logging data.(2) The mud shale in Ji yang depression is mainly composed of clay, sandstone, carbonate and organic matter. Determination of main mineral content is helpful to judge the fracturing of the shale. Based on the available conventional logging data, the establishment of a model to calculate the mass fraction of the main minerals is feasible.(3) Seismic reservoir prediction technology is also a key technology for the unconventional resources. It can solve problems such as the perdition of TOC content, evaluation of brittleness,the distribution of overpressure and crack,and so on. Due to the limitation of time, take a few key wells in Ji yang depression as an example and focus on the geophysical responses when the thickness and organic matter content of the shale varies.The geophysical method for prediction of "sweet spot" of shale oil has not formed a completed system, especially in the pre-stack seismic aspect.