Study Of Carboniferous Volcanic Reservoirs In Hongshanzui Oilfield Of Junggar Basin

Volcanic rocks are distributed in almost all basins as a base.Volcanic activities have multiple effects on petroleum reservoirs such as destruction,alteration and reconstruction.Over the past few decades,lots of prolific volcanic reservoirs have been discovered in hydrocarbon-bearing sedimentary basins.During the International Conference on Terrigenous Volcanics in May,2006,Jiaqi Liu,an academician of the Chinese Academy of Sciences,advocated that the exploitation of petroleum in volcanic rocks would be the third innovation in petroleum industry.It is anticipated that petroleum exploration and development in volcanic rocks are very promising and will be very important to increase petroleum reserves and production.Volcanic reservoirs are very complex because of their distribution style and heterogeneity.Our limited knowledge in volcanic reservoirs and the difficulties in objective evaluation of these reservoirs lead to undesirable development.The key factors of volcanic reservoir development include how to describe reservoir distribution in detail and how to analyze controlling factors profoundly.This study focuses on Hongshanzui volcanic reservoirin the Jungar Basin.The subtle structures have been interpreted using high resolution 3-D seismic data in the new exploration areas.Comprehensive technological routes integrated with multi-disciplines have been made based on the other work,such as volcanic strata division and correlation, lithology,lithofacies and crack reorganization,pore space types and association of reservoir,diagenesis evolution of reservoir,controlling factors of reservoir,comprehensive assessments of reservoir and geology modeling of reservoir,and so on.Lithology of volcanic reservoir, lithofacies,pore,crack,diagenesis,and geological modeling have been comprehensively studied in this paper.The drilling positions of appraisal wells have been recommended based on detailed description of reservoir distribution,accumulation condition,trap types,accumulation laws and controlling factors.This study shows that there were three key controlling factors for petroleum reservoir in the Hongshanzui oilfield in Junggar basin.The first factor is lithofacies that is the basis for the reservoir properties of volcanics.Recognition methods of volcanic reservoirs are optimally selected in this study.The Fisher method uses the differences of multiple variables in several major discriminant functions based on projection theory.The probability of recognition tends to be high through lithology reorganizations of four subclasses and nine subgroups.On this basis, volcanic lithofacies have been studied.The lithofacies of study consist of invasion facies,effusive facies,burst facies and facies associated with volcanic and alluvial fan.The volcanic facies are distributed in most of study area.Effusive facies are distributed in the east,and volcanic facies and alluvial fan in the south.Andesite,tuff and tuffaceous sandy conglomerate are prone to forming secondary pore and crack,and those are favorable reservoir,mainly distributed in east and southeast.Secondly,fractures formed by structure activity have significant effects on volcanic reservoir quality.Subtle 3-D seismic data show that faults formed during C-P are critical to improve reservoir space and faults formed during T-J are important to form high-angle fractures and miters.Fracture system is very important in controlling petroleum accumulation because fracture system controls the formation and distribution of secondary pore space and also acts as oil and gas migration pathways.Schematic models of fracture porosity and angle have been established and used in the interpretation of 44 wells in this study.Our study shows that the fracture is characterized by predominance of structural fractures,secondary diagenesis fractures and minor dissolving fractures.In addition,andesite and tuff are main reservoir rock,and tuffaceous conglomerate and tuffite are minor,volcanic breccia is the poorest.The fractures are mainly miter and netted fracture in attitude,and high angle fractures are minor, horizontal fractures are poorest.The later are the main fractures,which have not been in-filled effectively.The fractures formed during middle and early periods have been filled mostly.The fractures in the section are mainly distributed in the region about 300m apart from weather crust, especially about 150m apart from weather crust.The fractures in the plane are distributed band-like in nearby large rapture.The fractures distributed in the nearby Hong56A well have a trend about 25°,330°nearby Che72 well,and 300-340°nearby Hong18 well.The trends of fractures are consistent with major fractures formed during T-J.Dissolution is another important factor to improve the quality of fracture reservoirs.Volcanic reservoirs are characterized by lower porosity and extra-lower permeability in study area.Dissolution plays an important role in improve the volcanic reservoir quality.Dissolution occurs in the following conditions:(1) Good material base:volcanics mainly consisted with andesite and volcanic clastics,which were material base for forming secondary resolved porosity;(2) Sufficient fluid media such as(â…°) inorganic acid which is formed by the following ways,e.g.,forming by CO₂,SO₂,H₂S that are released during volcanic eruption and risen up from the deep crust through discordogenic fault(this kind of inorganic dissolution happens along the discordogenic fault),(â…±) organic acids which come from thermal evolution of organic matter and thermal decomposition of kerogen from the Permian source rocks with high TOC and Late Triassic source rocks with oil-prone kerogen having good hydrocarbon-generating potential(e.g.,Baijiatan formation);(3) Multi fluid pathways:the main fluid pathways are unconformity,regional crack and secondary faults and fractural reservoirs.The main types of dissolution are:(â…°) glassy matrix dissolution,(â…±) feldspar dissolution around the cleavage fissure,(â…²) feldspar dissolution around the fringe, (â…³) debris dissolution,(â…´) grain dissolution,(â…µ) calcite re-dissolution after filling the fracture,and(â…¶) secondary porosity and cavity from dissolution of grains and matrix.The dissolution has significant effect on improvement of crack reservoir in the study area.In general,dissolution occurs mainly around the regional fractures and secondary faults.This study on oil and gas reservoir accumulation shows that the oil generated from the Permian Fengcheng and Wuerhe formation accumulated in favorable structure trap and stratigraphic trap with good fracture and resolved porosity through the fault,unconformity and fracture reservoir.The play is a typical new source and old bearing pool. The main controlling factors of oil accumulation are:(â…°) the crack and unconformity controlling hydrocarbon migration,accumulation and redistribution;(â…±) the local seals controlling the distribution,abundance and scale of petroleum reservoirs,and(â…²) favorable lithofacies(or sedimentary facies) controlling the scale of petroleum reservoirs.Significant breakthrough has been accomplished in oil production from the Hong 019 Well that was recommended to due to improvements in our knowledge in geology.The Hong 019 well proposed for drilling had produced oil 110t and gas 2.527×10⁴m³ based on the well test using 5mm choke in Carboniferous,which had significant impact on economic gains.