Hydrocarbons in igneous rock of Brazil:A review

Igneous rocks are widely developed in many hydrocarbon-bearing sedimentary basins in Brazil,and some igneous rocks play positive roles in the oil and gas accumulation process.But so far,no industrial oil or gas flow is discovered in igneous rocks in Brazilian onshore basins.At present,in some literatures published by some researchers,cases of igneous reservoirs in Brazil are inaccurate.The lithology of reservoirs in the Igarape Cuia oil and gas field,the oil and gas field of Urucu and the Barra Bonita gasfield,were previously thought to be volcanic rocks,but now are clastic rocks according to studies,and the igneous rocks in these three oil and gas field can only be acted as indirect and direct cap rocks.Furthermore,igneous rock reservoirs in the Campos Basin and Santos Basin in the Brazilian offshore are briefly analyzed.The representative Badejo oilfield in the Campos Basin has igneous rock reservoirs dominated by basalts which are intercalated with thin layers of volcaniclastic and sedimentary rocks,the storage space are vesicules,fractures and vugular pores,and the porosity of hydrocarbon bearing interval basalt reservoirs ranges from 10%to 15%.In addition,for the Santos Basin,igneous rocks in this basin are dominated by basic rocks,some alkaline intrusive rocks develop good micro spaces,mainly crystal moldic pores,inter-and intra-crystal dissolution pores and fractures,better reservoir intervals are often concentrated such as at sections near to unconformity surfaces and can have porosity of 6%e9%or higher;local pre-salt igneous rock reservoirs in the Santos Basin have the necessary conditions for hydrocarbon accumulation and it will be a new target layer for hydrocarbon exploration.

Characteristics of the Cretaceous Magmatism in Huanghua Depression and Their Relationships with Hydrocarbon Enrichment

The Huanghua depression located in the hinterland of the Bohai Bay Basin in eastern China is a typical area for the research of multistage magmatic activities with hydrocarbon enrichment,many high-yield wells related to igneous rocks were discovered within the Cretaceous strata in recent years.However,the spatial and temporal distribution of Cretaceous igneous rocks remains unclear,and the relationships among magmatic sequence,lithology,and hydrocarbon enrichment have been poorly studied.In order to solve these problems,core observation,logging analysis,major element analysis,zircon U-Pb chronology,oil-bearing grade statistics and reservoir spaces analysis were used to subdivide the magmatism cycles and to investigate the characteristics of igneous reservoirs.Our results show that the Mesozoic magmatism in Huanghua depression started in 140.1±1.4 Ma and could be divided into two stages including the Early Cretaceous stage and Late Cretaceous stage.The Early Cretaceous magmatism principally developed basic-intermediate rocks in the north zone,and could be subdivided into 3 cycles with their forming ages of 140,125–119,and 118–111 Ma,respectively.By contrast,the late stage mainly developed intermediate-acid rocks centralized in the south zone and formed at 75–70 Ma.The GR and SP curves are good indicators for the contrast of different lithologies,lithofacies and for magmatic sequences division.Intensive magmatism may have an advantage to form reservoirs,since basalt in cycle III in the Early Cretaceous and dacite porphyrite in the Late Cretaceous have great exploration potential.Lithology and tectonic fractures have an important influence on the formation of reservoir spaces and hydrocarbon enrichment.The characteristics of Cretaceous magmatism and igneous reservoirs in Huanghua depression and adjacent areas are summarized,providing important information for igneous reservoirs research and oil-gas exploration in the Cretaceous in related areas.