Subaqueous volcanic eruptive facies,facies model and its reservoir significance in a continental lacustrine basin:A case from the Cretaceous in Chaganhua area of southern Songliao Basin,NE China

The conventional lithofacies and facies model of subaerial and marine pyroclastic rocks cannot reflect the characteristics of subaqueous volcanic edifice in lacustrine basins.In order to solve this problem,the lithofacies of subaqueous eruptive pyroclastic rocks is discussed and the facies model is established by taking the tuff cone of Cretaceous Huoshiling Formation in the Chaganhua area of the Changling fault depression,Songliao Basin as the research object.The results indicate that the subaqueous eruptive pyroclastic rocks in the Songliao Basin can be divided into two facies and four subfacies.The two facies are the subaqueous explosive facies and the volcanic sedimentary facies that is formed during the eruption interval.The subaqueous explosive facies can be further divided into three subfacies:gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies.The volcanic sedimentary facies consists of pyroclastic sedimentary rocks containing terrigenous clast subfacies.A typical facies model of the tuff cone that is formed by subaqueous eruptions in the Songliao Basin was established.The tuff cone is generally composed of multiple subaqueous eruption depositional units and can be divided into two facies associations:near-source facies association and far-source facies association.The complete vertical succession of one depositional unit of the near-source facies association is composed of pyroclastic sedimentary rocks containing terrigenous clast subfacies,gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies from bottom to top.The depositional unit of the far-source facies association is dominated by the subaqueous fallout subfacies and contains several thin interlayered deposits of the water-laid density current subfacies.The gas-supported hot pyroclastic flow subfacies and the pyroclastic sedimentary rocks containing terrigenous clast subfacies are favorable subaqueous eruptive facies for reservoirs in continental lacustrine basins.

Facies-controlled volcanic reservoirs of northern Songliao Basin, NE China

Volcanic rocks of the late Mesozoic are very important reservoirs for the commercial natural gases including hydrocarbon, carbon dioxide and rare gases in the northern Songliao Basin. The reservoir volcanic rocks include rhyolite, andesite, Wachyte, basalt and tuff. Facies of the volcanic rocks can be classified into 5 categories and 15 special types. Porosity and permeability of the volcanic reservoirs arc facies-controlled, Commercial reservoirs were commonly found among the following volcanic subfacics： volcanic neck （Ⅰ1）, underground-explosive breccia （Ⅰ3）, pyroclastic-bcaring lava flow （Ⅱ3）, upper effusivc （Ⅲ3） and inner extrusive ones （ Ⅳ1）. The best volcanic reservoirs arc generally evolved in the interbedded explosive and effusivc volcanics. Rhyolites show in general better reservoir features than other types of rocks do.

Subaquatic volcanic eruptions in continental facies and their influence on high quality source rocks shown by the volcanic rocks of a faulted depression in Northeast China

This paper reports the analysis on cores and rock slices, data on seismic and logging activities, characteristics of core samples, and the paleogeographic background of the Yingcheng Formation of the Xujiaweizi faulted depression in the Songliao Basin. The results show that some of the volcanic rocks were formed during subaquatic eruptions. These subaqueous volcanic rocks are further characterized by the interbedded black mudstone and tuffite, the presence of double-layer perlite enclosing aphyric or sparsely phyric rhyolite, the presence of a bentonite layer, and the coefficient of oxidation （Fe203/FeO）. The types of rocks are volcanic breccia, lava breccias, perlite, rhyolite, tuff and sedimentary tuff. The subaquatic eruptions are distributed mainly in Wangjiatun, Shengping, Xuxi, Xuzhong, and Xudong. The XS-I area is the most typical. The organic abundance of over- burden mud rocks within the volcanic rocks of the Yingcheng Formation indicates that these rocks represent high-quality source rocks. The analysis also shows that continental subaquatic volcanic eruptions provide a rich supply of minerals and en- ergies for the lake basin and increase the organic matter content in the water. Moreover, the water differentiation provides a good reducing environment for the conservation of organic matter, and is beneficial for the formation of high-quality source rocks. Finally, we propose a hypothesis to describe the mode of subaquatic eruptions and the formation of high-quality source rocks.

Volcanic Facies and Their Reservoirs Characteristics in Eastern China Basins

In these years, with more and more volcanic oil and gas fields being discovered and de- veloped, the volcanic rocks reveal a great petroleum potential in the eastern basins of China. There are five volcanic facies identified in the study area, which include volcanic conduit facies, explosive facies, effusive facies, extrusive facies, and volcanogenic sedimentary facies. The subaerial eruption usually happened in Mesozoic and Paleocene, and subaquatic eruption in Eocene. The upper subfacies and top autoclastic brecciation of effusive facies of subaquatic volcanic rocks and pyroclastic flow subfacies of explosive facies of subaerial volcanic rocks are the most favorable volcanic reservoirs. The intermittent belt formed between two times of volcanic eruptions is most effective for reservoirs both in subaquatic and subaerial volcanic rocks. Their main porosity types are interclast porosity, interflow laminar po- rosity, vesicular and gas pipes porosity, intercrystalline sieve of moldic porosity, secondary dissolution porosity, and tectonic fracture. Developed between pre-emplacement stage and final cooling, the pri- mary porosity may lead to high porosity and permeability, and the secondary porosity usually devel- oped upon them. The porosity of volcanic rocks was less influenced by the compaction and the burial depth.

Formation mechanism of in-situ volcanic reservoirs in eastern China:A case study from Xushen gasfield in Songliao Basin

A large number of in-situ volcanic reservoirs have been discovered from the Meso-Cenozoic rift basin group in eastern China.Based on drilling results in combination with geological and geophysical analysis,a case study from the Early Cretaceous Xujiaweizi fault-depression shows that the formation mechanism of in-situ volcanic reservoirs is characterized by"fault-controlled body,body-controlled facies,facies-controlled reservoir,and reservoir-controlled accumulation".In other words,deep faults control the volcanic eruption type,volcanic body,and gas reservoir distribution;the volcanic body determines the spatial distribution of volcanic facies and volcanic gas reservoir size;the volcanic facies control reservoir physical properties and effective thickness of gas formation;the volcanic reservoir properties control gas reservoir type and gas productivity.The result is useful to guiding the discovery of in-situ volcanic gas reservoirs in faulted basins in both theory and practice.

Seismic Reflection,Distribution,and Potential Trap of Permian Volcanic Rocks in the Tahe Field

Permian Kaipaileicike （开派雷兹克） volcanic rocks approximately 0-200 m thick are drilled in the Tahe （塔河） field. The distribution of volcanic rocks and their potential to form hydrocarbon reservoirs are discussed based on the integrated interpretation of log and 3D seismic data. The volcanic rocks, mainly consisting of dacites and basalts, are sandwiched between the Lower Triassic and Lower Carboniferous and bounded by top and bottom unconformities. The dacites accumulated in a mound shape around volcanic craters, whereas the basalts are deposited in tabular or trough-fill ge- ometries. Permian volcanic craters mainly located at the northwest corner of the Tahe field are identi- fied from volcanic rock thickening, occurrence of volcanic breccias, structural arch of the top Permian, seismic attribute anomalies, and fault （piercing conduit） reflections. Along the northwest wing of a Carboniferous salt dome, a stratigraphic trap is formed by a northeast updip pinch-out of Permian volcanic rock. Oil indications within the trap are found in numerous wells. The reservoir volcanic rocks are mainly of the fracture-pore pattern and covered by the caprock of a Lower Triassic mudstone. The hydrocarbon reservoir, which can potentially be a medium-sized oil pool, is connected to Camhrian-Ordovician source rocks through normal faults along the salt dome boundary.