Chang-Zhi Wu

Formation mechanisms of hydrocarbon reservoirs associated with volcanic and subvolcanic intrusive rocks: Examples in Mesozoic–Cenozoic basins of eastern China

Numerous petroleum-producing Mesozoic–Cenozoic basins are present in the coastal areas of eastern China. Voluminous volcanic and subvolcanic intrusive rocks, dominantly basaltic and, to a lesser degree, trachytic in composition, are intercalated or intruded in the sedimentary sequences. These magmatic rocks can serve as cap rocks as well as reservoir beds for hydrocarbons. Hydrocarbon reservoirs related to volcanic rocks can be classified into the volcanic-trapped type, the volcanic-sealed type, and the weathering crust type, and those related to subvolcanic intrusions include the doming-derived fracture type, the cryptoexplosive breccia type, the primary fracture type, the alteration zone type, the contact zone type, and the laterally sealed type. A generalized model for the formation of hydrocarbon reservoirs related to volcanic and subvolcanic rocks is proposed. We call for more attention to volcanic- and subvolcanic-related reservoirs during the exploration for hydrocarbons.

Hydrocarbon reservoirs in a trachyte porphyry intrusion in the Eastern depression of the Liaohe basin, northeast China

Hydrocarbon reservoirs in the Oulituozi area of the Liaohe basin, northeastern China, occur within a subvolcanic trachyte porphyry intrusion. This intrusion was emplaced at a depth of less than 1 km during the Paleogene. The prevalent reservoir porosity is in explosive breccias, fracture networks, steeply dipping cracks, and dilational microfractures in the apical zone of the intrusion. These voids were formed by underground explosion during magma solidification and by postsolidification volume contraction. Minor oil and gas volumes are trapped also in fractures in the overlying brittle beds. The massive rocks in the lower part of the porphyry body serve as hydrocarbon seals for reservoir beds lateral to the intrusion. We propose a model for reservoir formation related to a subvolcanic intrusion that includes three types of reservoirs: (1) reservoirs in the apical zone of the intrusion, (2) reservoirs in the upper wall of the intrusion, and (3) reservoirs lateral to the intrusion. Attention should be paid to all the three reservoir types during exploration around a subvolcanic intrusion.

Comparative study of ore-forming fluids of hydrothermal copper–gold deposits in the lower Yangtze River Valley, China

Fluid-inclusion and stable isotope studies were carried out on five types of Mesozoic (Yanshanian) hydrothermal copper–gold deposits in the lower Yangtze River Valley. Deposits include (1) copper in cryptoexplosive breccia pipes, (2) skarn copper, (3) porphyry copper, (4) high-temperature quartz vein-type copper and gold, and (5) medium–lower temperature fracture zone gold. This research has allowed a comparison between various types of ore-forming fluids. Melt-fluid inclusions in garnet from the matrix of the breccia pipe at the Shizishan copper deposit reveal the existence of a water-rich magma. In all deposit types, fluid temperatures and salinities were higher at early stages and generally decreased with time. Magmatic water is dominant in the high-temperature ore-forming fluids, whereas meteoric water was involved only in the medium–lower temperature Xiaomiaoshan gold deposit and in the post-mineralization stage of the Shaxi porphyry copper deposit. Fluid boiling played an important role in the mineralization of most deposits, particularly at Shizishan, where multi-stage boiling was associated with the formation of cryptoexplosive breccia, skarn, quartz-sulphide, and quartz-carbonate-sulphide stages. Boiling of an aqueous magmatic fluid system at high temperatures reflects the release of crystallization heat and increase of total volume of the magma–fluid system, and hence it can be referred to as active boiling. On the contrary, boiling of a fluid at lower temperatures is typically triggered by pressure release due to fracturing or dilation in the surrounding rocks, and is thus referred to as passive boiling. In general, passive boiling occurs more commonly at the higher levels of a hydrothermal mineral system and at later stages of the ore-forming process.

Transition from platemargin to intraplate environment: Geochemistry of basalts in Paleogene Liaohe basin, northeastern China

Paleogene basalts from the Liaohe basin, northeastern China, are dominated by alkaline olivine basalts and olivine basalts. These basalts are generally enriched in high field strength elements (HFSE), depleted in large ion lithophile elements (LILE) and comparable to those of typical ocean island basalts (OIB). Positive anomalies of Ba, Sr and Zr with high Nb/U, U/Pb, Ce/Pb and Zr/Hf ratios imply that materials from an oceanic crust had been added to the mantle sources of the basalts. In addition, the basalts are generally depleted in Sr, Nd and Pb isotopes, indicating that an enriched mantle (EMI) and a depleted mantle (DM) sources were added to the OIB-like resource.Comprehensive research on lithosphere evolution and tectonic setting of the Liaohe basin and surrounding areas suggests that these basalts were derived by variable degrees of partial melting from an upwelling asthenosphere mantle. Materials from an oceanic lithosphere were added to the source in company with Paleogene tectonic transition from platemargin to intraplate environment. Retreating and steepening of the subducting Pacific oceanic plate could be the main cause for the tectonic environment transition.

Sub-volcanic emplacement of the Oulituozi trachyte porphyry in the Liaohe Basin and its significance to Hydrocarbon reservoirs

The Oulituozi trachyte porphyry in the Liaohe Basin, NE China, was emplaced under the sub-volcanic environment. Hydrocarbons were trapped principally by crypto-explosive breccias, fracture networks, vertical extension fractures and tensile micro-fissures in the top portion of this intrusion.