Hydrocarbon accumulation mechanism and structure of large-scale volcanic weathering crust of the Carboniferous in northern Xinjiang, China

Abstract

The Upper Carboniferous in northern Xinjiang, China was formed in a post-collisional depression and collapsed structural setting. Within the Upper Carboniferous, volcanic rocks and source rocks alternate over a wide region. At the end of the Carboniferous, these layers were uplifted by plate collisions and subsequently weathered and leached. Volcanic weathering and leaching led to the establishment of weathered crusts that can be divided into five layers. Corrosion and crumble zones in these layers form favorable reservoirs. Volcanic weathering crust formed in sub-aerially exposed paleogeomorphic areas; the five relatively continuous layers are generally preserved in paleogeomorphic lowland and slope regions, but the upper soil layer is usually absent in structurally higher parts of the rock record. The thickness of the weathered layer has a positive nonlinear exponential relationship to the duration of weathering and leaching, and the dynamic equilibrium time of weathered crust is about 36.3 Ma. The thickest weathered layer (~450 m) is located in fracture zones. Weathered crusts are possible from a range of volcanic rocks with different lithologies, given sufficient time for weathering and leaching. The combination of volcanic weathered crust and source rocks results in three types of hydrocarbon accumulation models: (1) sequences of volcanic weathered crust interbedded with source rocks, (2) a quasi-layered weathered volcanic core located above source rocks, and (3) volcanic rocks associated with pectinate unconformities adjacent to source rocks. Each of these three types has the potential to form a giant stratigraphic reservoir of volcanic weathered crust. This knowledge has changed the traditional exploration model of searching for favorable lithologic and lithofacies zones in volcanic rocks, and has changed the viewpoint that the Carboniferous does not have the genetic potential to be the basement of the basin in northern Xinjiang. The concepts developed here are of great scientific significance and application for focusing oil and gas exploration on volcanic weathered crust. As such, the Paleozoic volcanic weathered crust in the midwestern part of China may possibly contain large-scale stratigraphic reservoirs and thus could be a new oil and gas exploration target in the future. Carboniferous in northern Xinjiang, volcanic weathering crust, stratigraphic reservoir, accumulation mechanism and model Citation:Zou C N, Hou L H, Tao S Z, et al. Hydrocarbon accumulation mechanism and structure of large-scale volcanic weathering crust of the Carboniferous in

Occurrence and Origin of H2S from Volcanic Reservoirs in Niudong Area of the Santanghu Basin, NW China

“…The basin in an area of 2 3 × 10 4 km 2 and with accumulative petroleum reserves of more than 3 × 10 8 tons has been discovered at present. Since the initial formation in the Silurian period, this basin has experienced several tectonic movements, inducing well-developed regional faults and strongly weathered local rocks [23][24][25]. Volcanic eruptions frequently occurred in and around the basin during the late Palaeozoic era which probably produced some fracture channels by faulting.…”

“…The volcanic rocks of the Haerjiawu Formation alternate with source rocks. Hydrocarbons were generated from these source rocks, migrated into the weathered volcanic crust directly or through the faults, and then accumulated to construct the petroleum reservoir in the study area [23].…”

“…The reservoir rocks of the Santanghu Oilfields are typical volcanic rocks of the Carboniferous Kalagang Formation, which are dominated by basalt, andesite, volcanic breccia, and tuff [23]. The SiO 2 content of the volcanic rocks is 44% to 65%.…”

“…With the progression of petroleum exploitation, unconventional oil and gas resources are the vital sources for exploration. Besides clastic and carbonate rocks, volcanic rocks as reservoirs are a potential target for oil and gas exploration as well in recent times [23]. Therefore, in the current scenario, the occurrence and origin of H 2 S in volcanic reservoirs should be an important topic for exploration and as a supplement to the basic theory.…”

Structure of weathered clastic crust and its petroleum potential

“…Petroleum geologists are interested in the physical properties of paleo-weathered crust with respect to their effect on reservoir quality and accumulation mechanisms. Most petroleum geologists associate weathered crust with unconformities, consider weathering as improving reservoirs properties, and classify the structure of weathered crust structure of carbonate, volcanics, and metamorphic rocks under the aspects of porosity, permeability, and microfracture development (He, 2002;Lü et al, 2008;Liu, 2010;Zou et al, 2011Zou et al, , 2012Hou et al, 2012). The role of weathered clastic crust as reservoir rocks has been rarely studied systematically and the focus is rather on lithology and stratal contact relation (Wu et al, 2003).…”