塔河地区中下奥陶统储层硫化物成因分析

研究表明塔河地区中下奥陶统碳酸盐岩储层后期受到了大气淡水和深部热卤水的成岩改造作用。这些储层天然气中含有高达8.3%H2S气体,裂缝与孔洞充填方解石流体包裹体中气相组分含有高达11%的H2S。这些方解石的均一化温度以110.2～198.9℃为主,而且,H2S气体、原油和黄铁矿集合体δ34S值主要介于18‰～22‰,这些特征显示,硫化物形成于相对高温条件下热化学硫酸盐还原-有机质氧化作用(TSR)。有机质被氧化的证据包括高温方解石具有轻δ13C特征(δ13C为-4.3‰～-8.3‰)以及现今地层水具有轻δ13CHCO3-值(-6.0‰～-13.8‰)。现今油气藏中TSR成因H2S浓度低于流体包裹体,应该与H2S沉淀为黄铁矿、合并入原油中而导致富硫原油产生有关。一些黄铁矿具有很轻的δ34S值,可轻达-26‰,为微生物硫酸盐还原成因,但是其分布比较局限。

Origin of sulfides in the Middle and Lower Ordovician carbonates in Tahe oilfield, Tarim Basin.

Middle and Lower Ordovician carbonate reservoirs in Tahe area,northern Tarim Basin,NW China,are shown to have been diagenetically altered by meteoric water and hot brines from the deeper underlying strata.Some of the reservoirs contain up to 8.3% H2S in natural gas composition and up to 11% H2S in gas-phase fluid inclusions in fracture-and vug-filling calcites,respectively.The host calcites have homogenization temperatures of fluid inclusions mainly from 110.2 to 198.9℃,and the H2S and pyrite aggregate have δ34S values mainly from 18‰ to 22‰.These features suggest that the sulfides may have generated by thermochemical sulfate reduction by organic matter(TSR).Oxidization of organic matter is evidenced by light δ13C values from-4.3‰ to-8.3‰ for the calcite and from-6.0‰ to-13.8‰ for present formation water HCO3-.The fact that H2S concentrations in the associated gas are significantly lower than those of fluid inclusions may indicate that the H2S may have precipitated as pyrite,and/or incorporated into crude oils to generate sulfur enriched oils.Some pyrite shows light δ34S values as low as-26‰,indicating a result from bacterial sulphate reduction.However,distribution of pyrite of BSR origin is relatively limited.