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Junggar Basin is one of the largest sedimentary basins in Northwest China. Carboniferous oil and gas fields have been found in different areas in the eastern part of the Junggar Basin on a large scale, indicating that the Carboniferous rocks of the Junggar Basin have a huge potential for oil and gas exploration. This study focuses on the Batamayineishan Formation in the eastern part of the Junggar Basin, which contains volcanic rocks and pyroclastic rocks, aiming to investigate the reservoir characteristics and to identify the formation mechanism of the rocks of this formation. The majority of the existent reservoir space in the volcanic rocks of the Batamayineishan Formation is dominated by secondary pores and fractures. Using the methods of petrography, pressure-controlled mercury injection (PMI), and electron probe microanalysis (EPMA), the reservoir characteristics and diagenetic history of the volcanic rocks of the Batamayineishan Formation in the Shuangjingzi area were studied. A theoretical framework is established to provide favorable guidance for exploring Carboniferous volcanic rocks in the Junggar Basin. The results of mercury injection indicate that the average pore throat radius and porosity of the volcanic rocks are 0.068 µm and 6.62%, respectively. Permeability remains stable and does not show a significant change with an increase in porosity. Despite the high porosity, the permeability is relatively low, reflecting isolated and non-connected primary pores. The average value of permeability is relatively low (0.424×10-3 µm2), which typically suggests narrow micro-throats. Primary gas pores fill and develop amygdales on a large scale. In addition, the dissolution pores developed by dissolution and alteration also compensated for the decrease in the original gas pore volume., Bazen Junggar jedan je od najvećih taložnih bazena u sjeverozapadnoj Kini. Karbonska ležišta nafte i plina otkrivena su u različitim dijelovima na istoku toga bazena te pokazuju kako karbonske stijene i dalje imaju velik potencijal za istraživanje ugljikovodika. Studija je usmjerena na istočni dio toga bazena i formaciju Batamajinejšan, koja je izgrađena od vulkanskih i piroklastičnih stijena. Istražena su njihova ležišna svojstva i način oblikovanja tih stijena. Najveći dio ležišnoga volumena obilježen je sekundarnom šupljikavošću i pukotinama. Tijekom istraživanja korištene su petrografske metode, utiskivanje volumena žive pod kontroliranim tlakom, mikroanaliza elektronskom sondom. Na taj su način ispi tana dijagenetska povijest i svojstva vulkanita formacije Batamajinejšan. Time je postavljen teorijski okvir svih istraživanja karbonskih vulkanskih stijena u tome bazenu. Utiskivanjem žive izračunan je prosječni promjer šupljina od 0,068 µm i šupljikavost 6,62 %, međutim propusnost je općenito niska (0,424 × 10-3 µm2) jer je dominantno rezultat male primarne, efektivne šupljikavosti te se znatno ne mijenja s povećanjem ukupne šupljikavosti, koja i dalje upućuje na mikropore. Sekundarna šupljikavost također je oblikovana i otapanjem, što je, uz pukotine, povećalo dostupan volumen za nakupljanje plina.

It has long been considered that thicker volcanostratigraphic sequences should, in theory, contain more favourable reservoir conditions than in thinner sequences, particularly within large-scale stratovolcanoes found in sedimentary basins. However, drilling results from the Wangfu Rift Depression (WRD) in the Songliao Basin, China, show that the thick intermediate volcanostratigraphic sequence contains only one or two layers of favourable reservoir characteristics over approximately 1000 m of volcanic successions. The volcanostratigraphic sequence that is common in the graben basin has a particular distribution pattern as a result of reservoir and origin processes. The Eruptive Interval Unconformity Boundary (EIUB) is introduced and the burial history is investigated in order to determine the reservoir characteristics and origin. We also identify what controls the gas pool accumulation in the thick intermediate volcanostratigraphy. According to the reworked volcanic rocks and weathered crust patterns, the volcanostratigraphy of the basin has 13 wells containing one EIUB, and 2 wells with two EIUBs. The exposed and burial histories of the volcanostratigraphic sequence have produced a variety of pore space including primary pores, secondary pores and secondary fractures. The secondary pores are the most dominant, followed by primary pores and finally fractures. These pore spaces form a reservoir with moderate to low porosity and permeability and the most productive reservoirs are located 40–80 m below the EIUB. The secondary pores are primarily controlled by weathering and leaching as well as deep-burial alteration. Due to the migration of fluids from the overlying strata into the volcanic rocks of the Huoshiling Formation below, the top most layer dissolves first, which significantly enhances the secondary porosity of this layer found 40–80 m below the EIUB. Thus, the number of EIUBs should control the number of favourable reservoir layers. Based on analysis of the reservoir, source and cap rocks in the study area, we conclude that the best exploration targets in these thick volcanostratigraphic sequences, specifically in half-graben basins, are the volcanic inherited-uplift slope areas, followed by the inherited-uplift of the sag area and the uplifted zones. The findings presented in this work provide an insight of the evolution of porosity in other volcanic systems elsewhere and contribute to the likelihood of locating the favourable exploration targets for thick volcanostratigraphic sequences in half-graben basins.

Just like in sedimentary stratigraphy, the volcanostratigraphic boundary is an important factor for constructing volcanostratigraphic framework. The fundamental factor of volcanostratigraphic boundaries is to classify the types and define their characteristics. Based on field investigation and cross-wells section analysis of Mesozoic volcanostratigraphy in NE China, 5 types of volcanostratigraphic boundaries have been recognized, namely eruptive conformity boundary (ECB), eruptive unconformity boundary (EUB), eruptive interval unconformity boundary (EIUB), tectonic unconformity boundary (TUB) and intrusive contacts boundary (ICB). Except ICB, the unconformity boundaries can be divided into angular unconformity and paraconformity. The time spans and signs of these boundaries are analyzed by using age data of some volcanic fields that have been published. The time spans of ECB and EUB are from several minutes to years. In lava flows, cooling crust is distributed above and below ECB and EUB; in pyroclastic flows, airfalls and lahars, a fine layer below these boundaries has no discernable erosion at every part of the boundary. EUB may be curved or cross curved and jagged. The scale of ECB/EUB is dependent on the scale of lava flow or pyroclastic flows. The time span of EIUB is from decades to thousands of years. There is also weathered crust under EIUB and sedimentary rock beds overlie EIUB. In most instances, weathered crust and thin sedimentary beds are associated with each other laterally. The boundary is a smooth curved plane. The scale of EIUB is dependent on the scale of the volcano or volcano groups. The characteristics of TUB are similar to EIUB's. The time interval of TUB is from tens of thousands to millions of years. The scale of TUB depends on the scale of the basin or volcanic field. Both the lab data and logging data of wells in the Songliao Basin reveal that the porosity is greatly related to the boundaries in the lava flows. There is a high-porosity belt below ECB, EUB or EIUB, and the porosity decreases when it is apart from the boundary. The high-porosity belt below ECB and EUB is mainly contributed by primary porosity, such as vesicles. The high-porosity belt below EIUB is mainly contributed by primary and secondary porosity, such as association of vesicles and spongy pores, so the area near the boundary in lava flows is a very important target for reservoirs.

Deep-large faults in the central continental margin of eastern China are well developed. Based on the regularity of spatial and temporal distribution of the faults, four fault systems were divided: the Yanshan orogenic belt fault system, the Qinling-Dabie-Sulu orogenic belt fault system, the Tanlu fault system and the East China Sea shelf basin-Okinawa trough fault system. The four fault systems exhibit different migration behaviors. The Yanshan orogenic belt fault system deflected from an EW to a NE direction, then to a NNE direction during the Indo-Chinese epoch-Yanshanian epoch. The thrust-nappe strength of the Qinling-Dabie orogenic belt fault system showed the tendency that the strength was greater in the south and east, but weaker in the north and west. This fault system faulted in the east and folded in the west from the Indo-Chinese epoch to the early Yanshanian epoch. At the same time, the faults also had a diachronous migration from east to west from the Indo-Chinese epoch to the early Yanshanian epoch. On the contrary, the thrust-nappe strength was greater in the north and west, weaker in the south and east during the late Yanshanian epoch-early Himalayan epoch. The Tanlu fault system caused the basin to migrate from west to east and south to north. The migration regularity of the East China Sea shelf basin-Okinawa trough fault system shows that the formation age became younger in the west. The four fault systems and their migration regularities were respectively the results of four different geodynamic backgrounds. The Yanshan orogenic belt fault system derived from the intracontinental orogeny. The Qinling-Dabie-Sulu orogenic belt fault system derived from the collision of plates and intracontinental subduction. The Tanlu fault system derived from the strike-slip movement and the East China Sea shelf basin-Okinawa trough fault system derived from plate subduction and retreat of the subduction belt.