Geochemistry of fluids from oil and gas fields and mud volcanoes in Taiwan

Abstract

Geochemical characteristics of fluids from oil and gas fields and mud volcanoes in Taiwan are summarized as follows.

Crude oils are dominated by condensate and light oil with high GOR, which clearly demonstrate the characteristics of coal-formed oils. API gravities of oils from Changkan（CBK）, Chingtsaohu（CTH）and Chinshui（CS）fields decrease with depth. Oils in the northern part are divided into two groups on the Ts /（Ts+Tm）－ C₂₉Sterane 20S /（20S+20R） plot, which are related with lithofacies of source rocks, and these two groups of oils, whose quality of organic matters and maturity are different, show the presence of different kitchen areas. Light hydrocarbons of oils from CBK, CTH and CS fields show evaporative fractionation, which is one of origins for condensate.

Hydrocarbon gases, partly generated from marine sources, are of thermogenic origin and two types of gas, in-situ and migrated from deeper part, are mixed. Contents of CO₂ change with depth and by fields, and three groups,（A, B and C groups）, are recognized based on CO₂ contents and δ¹³C_CO2 . Although A group is of organic origin, a part of B and C group is of inorganic origin.

Cl^－ concentrations in formation waters from fields are less than 15,000 mg/l and those from CS and Tiechenshan fields decrease with depth. Fluids from mud volcanoes related with anticline show similar geochemical characteristics as formation waters from oil and gas fields in Japan and are derived from fossil sea water.