油气耗散--油气地质研究和资源评价的弱点和难点

油气生成后运移的结果有3种:1)聚集成藏;2)散于岩层和流体中;3)与途经的流体-围岩相互作用所耗损或暴露地表而散失,后者称之为"耗散"。在油气运移过程中,耗散部分最为活跃,与聚集成藏的油气联系最为密切;为油气地质研究、勘探部署和资源评价等不可或缺的重要研究内容。根据油气耗散前、后数量和性质等方面的差异,将油气耗散划分为数量减少、相态转变、成份改变和性质转化4种类型,分别不同程度地体现了油气耗散的环境、条件和特点等方面的差异。其中性质转化类通常发生在埋藏不很深的表浅层或地表,油气与围岩、流体或大气等相互作用所形成的各种还原蚀变等后生产物及矿产,一般已不具烃类的表观特征。但其油气耗散量通常十分巨大,应给以足够的重视。将这类一般不具烃类特征的产物与直接显示油气耗散的结果(如油气苗、沥青、稠油等)相结合,对其追根溯源,可能为探讨和解决油气耗散及其规模这一国际难题的重要途径;也是示踪、发现和预测相关矿床的有效线索和重要标志。本文探讨了鄂尔多斯盆地东北部与上古生界天然气耗散有关的各种后生产物的类型、基本特征、地球化学性质、形成环境和分布特点等。

Nutrient Release from Weathering of Purplish Rocks in the Sichuan Basin,China

Purplish soils having high fertility with mineral nutrients inherited from the parent rock are widely distributed in the hills along the Yangtze River,especially in the Sichuan Basin.Pot and field weathering experiments were conducted to mimic rock weathering and nutrient release processes in order to better understand soil fertility and nutrient compensation. Three types of purplish rock formations formed in the Jurassic period,Shaximiao(J_2s),Suining(J_3s),and Penglaizhen (J_3p),as well as one type formed in the Cretaceous period,the Chengqiangyan group(K_1c),were used in this study. Results showed that the soil formation rate was in the range from 11.2 to 19.6 mm every year,and rock weathering was in the order of J_3s>J_3p>J_2s>K_1c.Because more rock surface was exposed to sunlight and rainfall in field conditions,pot weathering was slower than field weathering.Nutrient release rates increased with rock weathering and was in the order similar to that of rock weathering:J_3p>J_3s>J_2s>K_1c.Potassium release was the most important in all rocks;after 2 years of weathering,19.4% to 46.9% of K was released from the initial parent rocks,which suggested that K release from weathering could meet most of the crop K requirement in purplish soils.Thus,rapid release of nutrients from weathering of purplish rocks was key to nutrient replenishment and fertility of purplish soils.

Non-Darcy flow seepage characteristics of saturated broken rocks under compression with lateral constraint

Using an MTS816.03 test system and self-designed seepage apparatus, seepage tests of saturated broken rocks were conducted, and the influence of lithology, axial stress, grain size distribution and loading rate on seepage characteristics was analyzed. The results show that： （1） Under the same axial stress （12 MPa）, the permeability of different lithologic samples increases in the order： gangue 〈 mudstone 〈 sandstone 〈 limestone. The permeability of gangue is 3 magnitudes lower than that of limestone. The absolute value of the non-Darcy coefficient β increases in the order： limestone 〈 sandstone 〈 mudstone 〈 gangue. The non-Darcy coefficient β of limestone, which is positive, is 5 magnitudes lower than that of gangue. （2） With increasing axial stress, the permeability of saturated broken sandstone decreases, and the absolute value of the non-Darcy coefficient β increases. After the axial stress exceeds 12 MPa, the curves of permeability and non-Darcy coefficient β all tend to be stable. （3） With increasing Talbol power exponent, the permeability increases, and the absolute value of the non-Darcy coefficient β decreases. （4） With increasing loading, the permeability increases, and the absolute value of the non-Darcy coefficient β decreases. When the loading rate is 0.5 kN/s, the non-Darcy coefficient β is positive.

Groundwater Dating and Its Application to Earthquake Monitoring

Monitoring and study of dynamic characteristics of groundwater are significant methods of earthquake monitoring and forecasting. For research on groundwater dynamics,groundwater dating can qualitatively and quantitatively provide scientific analysis on the characteristics of groundwater recharge and runoff as well as renewal capacity. This article illustrates the methods used globally and summarizes the main advances and achievements in groundwater dating. It also focuses on the relationships between groundwater renewal capacity and seismic monitoring,groundwater movement and seismic activity,shallow groundwater recharge and abnormal interference elimination. The studies show that groundwater dating plays an important role in water-rock interaction,and geological tectonic and seismic activity evaluation. Therefore,groundwater dating can be widely used to monitor and analyze the precursor information in seismic underground fluid observations in the near future.

Diagenetic fluid evolution and water-rock interaction model of carbonate cements in sandstone: An example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area, Sichuan Province, China

Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolu- tion, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios （δ13C = 2.14%o, 8180 = -5.77‰）, and was precipitated early. It was precipitated di- rectly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid（ δ180 = -3‰）, which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios （δ13C = -2.36%0, 8180 = -15.68‰）. This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2＋, Fe3＋ and Mg2＋ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the buri- al process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite ce- ments in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone （δ13C = 1.93%o, δ180 = -6.11‰）, demonstrating that the two are from the same source that had a heavier oxygen isotope ratio （δ180 of about 2.2‰）. The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different wa- ter-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles （lithic quartz sand- stone） has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.