Volcanic Reservoir Geological Characterization Of The Batamayineishan Formation In Junggar Basin

Since 1990s, Great achievements on hydrocarbon exploration in volcanic rocks have been made in the Batamayineishan formation, upper Cretaceous, Junggar basin. Several large scales of oil & gas fields, such as Shixi oilfield and Wucaiwan gas field, have been built up to produce hydrocarbon hosted in volcanic reservoir. One of the most urgent tasks for further effective hydrocarbon exploration is to clarify the entire eruptive sequence of Batamayineishan formation and offer a standard for inter-well volcanic sequence comparison. There is no single petroleum drill which can acts as the standard sequence within the Junggar basin yet. There are continuous and entire outcrop sections of the Batamayineishan formation in the boundary area of the Junggar basin. These sections are perfect natural laboratories for volcanic reservoir geological study.Typical outcrop sections of the Batamayineishan formation are the strato-type section, Shuangjingzi section in Qitai, Jinshangou section in Qitai, Zhifang section in Balikun, and Baijiangou section in Jimusar. The total stratigraphical thickness of those outcrop sections is about 13,000m. Volcanic reservoir geological studies on the lithology, lithofacies, reservoir space, and geological features, have carried out in the continuous sections. The entire eruptive sequence of Batamayineishan formation is established by inter-section correlation. By comparing the petroleum drills to the standard sequence, the preferential volcanic targets for hydrocarbon exploration within the basin can be sketched out.1. Entire eruptive sequence of the Batamayineishan formationThe volcanic eruptive sequence is subdivided into formation, member, eruptive cycle and eruptive stage. Single eruptive cycle or stage is bounded with typical geological interfaces that include wreathing crust, thin sedimentary bedding, unconsolidated layer and tuff layer. The internal eruptive process is characterized based on the lithological and lithofacies composition. The entire eruptive sequence of Batamayineishan formation is divided into four members and nine eruptive cycles. The sequence could act as the standard for petroleum wells comparison. The 1^st member is mainly composed of intermediate and basaltic volcanic rocks and includes four eruptive cycles. The lowest Cycle 1 is characterized by acid-intermediate lithological combination with a thickness of 150-600m. Cycle 2 is characterized by basaltic volcanic rock with a thickness of 150-350m. Cycle 3 is characterized by basaltic-intermediate-acid lithological combination with a thickness of 300-600m. The uppermost cycle 4 is characterized by intermediate-acid lithological combination with a thickness of 250-450m. The 2nd member is made up of two intermediate-acid lithological combinations which correspond to two eruptive cycles, Cycle 5 with a thickness of 550-990m and Cycle 6 with a thickness of 300-500m. The 3^rd member only include Cycle 7 which consists of acid volcanic rock with a thickness of ca. 500m. The 4^th member is mainly composed of intermediate-acid lithological combination and two eruptive cycles. Cycle 8 is composed of intermediate-acid lithological combination with a thickness of 300- 500m, Cycle 9 is composed of intermediate volcanic rocks with a thickness of 200-300m.2. Lithological and lithofacies characteristicsThe specific lithology of volcanic rock is named in the order of genetic mechanism, the content of SiO₂, and the mineral & structures. There are ca. 16 kinds of volcanic rocks have been identified in the outcrops. Based on the genetic mechanism, the volcanic rocks are classified as lava, welded pyroclastic rocks and pyroclastic rocks. The lave contributes the total thickness of ca. 72% and the rest contribute ca. 15%, respectively. Based on the content of SiO₂, the volcanic rocks are classified as acid, intermediate-acid, intermediate, intermediate-basic, and basic volcanic rocks. The intermediate volcanic rocks contribute the total thickness of ca. 64%, and the acid and basic rocks contribute ca. 18%, respectively. With respect to specific volcanic rock, the andesite is the most abundant with a contribution of 50%, the basalt, rhyolite and intermediate pyroclastics contribute ca. 10%, respectively.The volcanic lithofacies of the Batamayineishan formation is classified as"five lithofacies and twelve subfacies". Effusive facies and explosive facies contribute the total thickness of ca. 69.4% and 18.0%, respectively. The rest facies'contributions are less than 10%, respectively. With respect to the subfacies compositions, the contributions of lower subfacies and middle-upper subfacies are 48.8% and 20.5%, respectively. No other subfacies contribution exceeds 10%. 3. Volcanic reservoir space features and influencing factorsAccording to the genetic mechanism, the volcanic reservoir spaces of Batamayineishan formation are classified as primary space and secondary space. Each kind of reservoir spaces are composed of pores and fissures. The primary reservoir spaces consists of seven kinds of spaces including vesicular pore, pores within almond-shaped structures, pores within lithophysa, inter-pyroclastic spaces, shrinkage fissure, inner-phenocryst fissure, and shrinkage joint. The secondary reservoir spaces consists of five kinds of spaces including eroded pores within phenocryst, matrix, and almond-shaped fills, tectonic joints and weathered joints. The influencing factors of volcanic reservoir spaces include primary eruptive lithological and lithofacies characteristics, tectonic activities, deep hydrothermal fluids and volcanic edifice. The central assemblage of volcanic edifice collects all the beneficial factors for reservoir space development, hence, the central assemblage is the most important target during the modern hydrocarbon exploration in volcanic rocks. The geological features of central assemblage of volcanic edifice mainly include special structural units and petrological compositions. The normal structural units are ring faults, radial dikes, and columnar joints. The normal petrological compositions are crypto-explosive breccias and welded breccia/agglomerate lave of conduit facies, pearlite of extrusive facies, breccias/agglomerate of explosive facies, vesicular lave of effusive facies. Besides these, Sulfur deposits and hydrothermal activities could be observed by chance.4. Volcanic features of petroleum drills in Dixi upliftThe petroleum drills in Dixi uplift have discovered only lower units of the Batamayineishan formation today. The 1^st and 2^nd member remained and the 3^rd and 4^th member have not been found yet, correspondingly, the lower cycles from cycle 1 to cycle 6 are remained and the upper cycles from cycle 7 to cycle 9 have not been found yet. The pyroclastic rocks are dominating with 57% of total thickness, and the lava contributes 24% of total thickness. With respect to the lithofacies composition, the explosive facies contributes about 51%, and the conduit facies, volcano- sedimentary facies, effusive facies contribute 17%, 15% and 13%, respectively, to the total thickness of the Batamayineishan formation in Dixi uplift.5. Correlation of volcanic characteristics between outcrops & petroleum drills and its significance of volcanic reservoirIt is the first practice that to restore entire eruptive sequence of the Batamayineishan formation. The standard eruptive sequence will play a key role during the stratigraphic correlation within the Junggar basin, and sketch out the geological targets for hydrocarbon exploration. Compared with the outcrops, the upper units of the Batamayineishan formation have not been found, the lithology is pyroclastic rock dominating and the lithofacies is explosive facies domination. According to the statistical results of surface porosity, the volcanic rocks with better porosity and permeability have the following characteristics in general. The lithology belongs to intermediate-acid lave with vesicular structure and basaltic breccia lava, the lithofacies belongs to middle-upper subfacies of effusive facies, pyroclastic flow subfacies and proximal air-fall subfacies of explosive facies, the beneficial position is upper units of eruptive cycle and central assemblage of volcanic edifice. Based on the volcanic reservoir geological characteristics, and taking into account the loss of the upper units of the Batamayineishan formation, the target of hydrocarbon exploration within the Junggar basin could be summarized as following combination. The ideal combination is the central assemblage of lava edifice in lateral, the upper units of eruptive cycles in vertical, the vesicular lava and agglomerate lava of effusive and conduit facies as the reservoir.