川东-渝北地区黄龙组白云岩储层稀土元素地球化学特征

Geochemical Characteristics of Rare Earth Elements of Huanglong Formation Dolomites Reservoirs in Eastern Sichuan-Northern Chongqing Area

石炭系黄龙组白云岩储层是川东—渝北地区天然气藏最重要的储集岩类型之一,依据岩石学特征和沉积相及地球化学特征,可将川东—渝北地区石炭系广泛发育的白云岩划分为准同生白云岩、成岩埋藏白云岩、古表生期淡水白云石和深埋藏热液异形白云石4种成因类型,其中具备储集意义的仅为成岩期交代成因的埋藏白云岩和古表生期形成的岩溶岩,岩性主要为颗粒和晶粒白云岩及白云质岩溶角砾岩。白云岩稀土元素地球化学分析结果为①弱溶蚀、强溶蚀准同生和成岩埋藏白云岩具有相似的ΣREE值、ΣLREE/ΣHREE比、REE配分模式和δCe、δEu负异常,但弱溶蚀白云岩各参数值相对较低;②淡水白云石具有很低的ΣREE值、弱Ce负异常和中等—较强的Eu负异常;③热液异形白云石ΣREE、REE配分模式和δCe特征与淡水白云石相似,而δEu最高。上述特征进一步表明各成因类型白云岩的稀土元素各项参数和配分模式差异明显,与较晚期次流体的物理化学性质密切相关,特点为准同生期和早—中成岩期埋藏白云岩受表生期岩溶作用影响较大,而深埋藏期热液异形白云石明显叠加有深部热液流体的影响。

The dolomite reservoir of Upper Carboniferous Huanglong Formation is one of the most important natural gas reservoirs in Eastern Siehuan--Northern Chongqing area. Based on the lithology characteristics of dolomites and combined with the palaeogeographical setting of the study area, four kinds of dolomite types：the penecontemporaneous dolostone, the diagenetic burial dolostone, the fresh water dolomite of epidiagenesis and hydrothermal dolomite of late diagenetic stage have been recognized. The burial diagenetic dolostone with diagenetic metasomatie geneses only are of reservoir significance,including two main rock types of grain dolostone and crystalline dolostone. By the synthetical analysis of geochemical characteristics of rare earth elements, it is recognized that different diagenetic dolostones are of various REE geochemistry characteristic and REE distribution patterns. This differences are connected with diagenetic liquid and physical chemistry characteristics of different phases, the features are as follows.（1）the penecontemporaneous dolostone and the burial diagenetic dolostone were much attected by karstification ot epidiagenetic stage, and the slightly corroded dolostones and the strong corroded dolostones respectively has similar REE geochemical characteristic; （2）the fresh water dolomite of epidiagenesis has lower ∑REE, weak Ce negative anomaly and moderate-strong Eu negative anomaly;（3）the hydrothermal dolomite of late diagenetic stage were affected mainly by deep hydrothermal liquid, which has similar REE geochemistry characteristic with the fresh water dolomite of epidiagenesis.