Volcanic Reservior Characteristics And Distribution In Santanghu Basin

In1990s,good oil and gas shows were firstly discovered in volcanic rocks of Tiaohu Fmin Tc1, T1etc,Santanghu basin. In2006and2008, oil fields were successively discovered inKaragang Fm. and Harjiawu Fm. As more and more oil fields were discovered in volcanicrock reservoir of the three formations,it suggests great exploration potential in Santanghubasin.This paper aims to study the characteristics of volcanic rock reservoirs and thedistribution of favorable reservoirs in Santanghu basin. Based on previous research resultsand new data,by thin section identification of volcanic rock specimens and logging analysisin Santanghu basin,I deeply studied the characteristics of the volcanic rock reservoirs, therelationship of diagenesis and pore evolution,and analyzed the controlling factors of volcanicrock reservoirs and the distribution of favorable reservoirs. These are the main contentsstudied in this paper. The target strata cover Tiaohu Fm（Middle Permian series）、KaragangFm（Lower Permian series）and Harjiawu Fm（Upper Carboniferous series）,in which oil andgas reservoirs of volcanic rock are found in Santanghu basin.1Lithologic features： The Lithologic features are similar among Tiaohu Fm,KaragangFm and Harjiawu Fm in Santanghu basin.although the proportion of each lithology isdifferent. The volcanic rock is mainly alkaline and calc-alkaline in Santanghu basin,making the volcanic rocks slightly alkaline in the mass. The Volcanic rocks is mainlyintermediate and basic rock, and most of them are lava, followed by volcanic clastic rock.The Volcanic lavas include basalt and andesite with fumarolic structure,amygdaloidalstructure or massive structure and breccia lava; volcanic clastic rocks include volcanicbreccia and tuff. Besides, there are some diabase and rhyolite.2Lithofacies features： The intermediate and basic volcanic rocks can be divided into four facies and even eleven subfacies in Santanghu basin. Both of Tiaohu Fm and Karagangare Fm are mainly effusive facies, followed by burst facies; most of volcanic rock inHarjiawu Fm are effusive facies and burst facies, with effusive facies slightly more than burstfacies, and volcanic sediment facies also develop well.3Volcanic Edifices characteristics： Volcano in study area is mainly shield eruptionvolcanic edifices. Volcanic Edifices in Karagang Fm and Tiaohu Fm may be fissure typeshield eruption volcanic edifices, with two row vocanic rock body which strike NW as twostring of beads. In contrast, Volcanic Edifices in Harjiawu Fm, in which craters are moredispersed, may be crater effusive shield eruption volcanic edifices. CNCF and PF arefavorable reservoir facies belts.4Reservoir space： reservoir space types of vocanic rock in Santanghu basin can bedivided into hole and crack. Hole class contains fumarole, amygdale and dissolution pore inmatrix, kinds of mineral crystals and clastic grains. crack class includes primary fracture(autoclastic crack and shrinkage joint) and secondary fracture(dissolution fractures,tectoclase). Pore-fracture type is the major combination type of reservoir space.5Diagenesis of Volcanic rock and pore evolution：the primary reservoir space in lava,and pyroclastic rocks was formed in early diagenesis period; but the primary reservoir spacewould be modified in late diagenesis period, so that primary reservoir space was filled orvolcanic rock was dissolved,. As a result, late diagenesis can both damage and improve thevolcanic rock reservoir. For volcanic rock of Harjiawu Fm, Karagang Fm and TiaohuFm(from Upper Carboniferous to Middle Permian series) in Santanghu basin, alteration andfilling effects are major diagenesis that damage reservoir space, in contrast, condensation,weathering and leaching, tectonism, dissolution with organic acid are most important forimproving volcanic rock reservoir property. The most important pore types are fumarole andamygdale formed by condensation and secondary pore formed by weathering and leaching orby dissolution with organic acid. Structural fractures play a role of communicating pores andpromoting dissolution.6Taking lithology, lithofacies and edifices features of volcanic rock, fracturedevelopment characteristic and the distribution of weathering crust into consideration, Ianalyzed the distribution of two kinds of favorable reservoir in the three formations. Thefavorable reservoir distribution zones for weathered and leached reservoir of Tiaohu Fm layin southern margin of Tiaoshan uplift and the southern thrust belt. That of Karagang Fm laybetween the denudation lines of Lucaogou Fm and Karagang Fm, and extend SW slightly somewhere, distributing as a belt in southern margin of Tiaoshan uplift. dissolved reservoirwith organic acid are distributed to the SW of weathered and leached reservoir, and can bedivided into two belts, one of them lays in southern thrust belt., the other lays near theweathered and leached reservoir. The reservoir type for Harjiawu Fm is dissolved reservoirwith organic acid, as its volcanic Edificess are dispersed, the favorable reservoir mainlydeveloped around T16/T11/M4well and Niuquanhu and Niudong area.