Hydrocarbon gases in Tertiary and Quaternary sediments offshore Peru

Hydrocarbon gases (methane, ethane, propane, isobutane, n-butane, ethene, and propene) are present in Tertiary and Quaternary shelf, upper-slope, and lower-slope deposits of the Peruvian continental margin. Methane dominates the composition of the hydrocarbon gas at all 10 sites examined during Ocean Drilling Program (ODP) Leg 112. Generation of methane is regulated by the amount of sulfate in pore water. Wherever sulfate concentrations approach or equal zero, methane concentrations increase rapidly, reaching values near 100,000 µL/L of wet sediment at eight of the 10 sites. Methane at all 10 sites results from methanogenesis, which is inhibited where sulfate is present and microbial reduction of sulfate occurs.
Hydrocarbon gases heavier than methane also are present, but at much lower concentrations than methane. These hydrocarbons are thought to result from early thermal and microbial diagenesis, based on relative gas compositions and trends of concentrations with depth.
With few exceptions, the results obtained in the shipboard and shore-based laboratories are comparable for methane and ethane in sediments of Leg 112. Reanalyses of canned sediments from ODP Leg 104 and from Deep Sea Drilling Project (DSDP) Legs 76 and 84 show that gas samples can be stored for as long as 8 yr, but the amounts of individual hydrocarbon gases retained vary. Nevertheless, the trends of the data sets with depth are similar for fresh and stored samples.

秘鲁大陆边缘的第三纪、第四纪陆架、上陆坡与下陆坡沉积层中赋存烃类气体，具体包括甲烷、乙烷、丙烷、异丁烷、正丁烷、乙烯以及丙烯。在大洋钻探计划（Ocean Drilling Program, ODP）第112航次所钻探的全部10个站位中，甲烷均为烃类气体组成中的优势组分。甲烷的生成受孔隙水中硫酸盐含量调控：当硫酸盐浓度趋近于或降至零时，甲烷浓度会迅速升高，在10个站位中的8个站位处，其浓度可达湿沉积物中约100000 µL/L。本次10个站位中的甲烷均源自产甲烷作用（methanogenesis），当孔隙水中存在硫酸盐并发生硫酸盐微生物还原作用时，产甲烷过程会受到抑制。
除甲烷外，重质烃类气体同样存在，但其浓度远低于甲烷。根据气体组分的相对占比与浓度随深度的变化规律，这类烃类被认为源于早期热成岩与微生物成岩作用。
绝大多数情况下，第112航次沉积物中甲烷与乙烷的船上实验室与陆上实验室分析结果具有可比性。对大洋钻探计划第104航次以及深海钻探计划（Deep Sea Drilling Project, DSDP）第76、84航次的封存沉积物进行的再分析结果显示，气体样品可被保存长达8年，但不同烃类气体的留存量存在差异。不过，新鲜样品与储存样品的数据集随深度变化的趋势整体保持一致。