渤海湾盆地歧口凹陷陆相湖盆页岩气富集条件及勘探潜力

Enrichment conditions and exploration potential of shale gas in continental lake basins in Qikou Sag,Bohai Bay Basin

为了进一步评价渤海湾盆地歧口凹陷陆相湖盆页岩气富集条件与勘探潜力,利用地震、测井、录井和岩心分析资料,对该区古近系沙河街组三段(以下简称沙三段)泥页岩的沉积环境、岩性及其组合特征、生烃条件、储集空间、岩石脆性、含气性等开展了综合研究,建立了该区页岩气的成藏模式,评价了页岩气的勘探潜力。研究结果表明:①该区沙三段泥页岩累计厚度大、分布面积广,在歧口凹陷的中—低斜坡区已进入大量生气阶段,具备形成规模页岩气藏的生烃条件;②该区泥页岩中长英质矿物含量占优势,基质孔隙、微裂缝较发育,具有一定的储集空间,同时脆性矿物含量高,黏土矿物中伊利石和伊/蒙间层的合计含量占94%,低膨胀性脆性页岩易于被压裂改造;③持续生烃过程中产生的流体膨胀是沙三段超压形成的主要原因,压力系数大于1.2的泥页岩段随钻气测异常明显,具备页岩气成藏条件;④凹陷内沙三段埋深介于3100~4400m,页岩气与页岩油伴生,主要以溶解气的形式存在;沙三段埋深介于4400~4850m,以湿气为主;沙三段埋深超过4850m,以干气为主。结论认为,歧口凹陷中—低斜坡区是页岩气勘探的有利目标区,有利勘探面积约1476km^(2),页岩气资源量超过1×10^(12)m^(3),具有较大的页岩气勘探潜力。

To further evaluate the shale gas enrichment conditions and exploration potentials of the continental lake basin in Qikou Sag of the Bohai Bay Basin,this paper studies the Es_(3)(the third Member of Paleogene Shahejie Formation)shale in this area comprehensively from the aspects of sedimentary environment,lithology and its combination characteristics,hydrocarbon generation conditions,reservoir space,rock brittleness and gas bearing property by using seismic,wireline logging,mud logging and core analysis data.In addition,the shale gas accumulation pattern in this area is established and shale gas exploration potential is evaluated.And the following research results were obtained.First,the Es_(3) shale in this area is of great cumulative thickness and large distribution area,and in the middle-low slope area of Qikou Sag it has entered the stage of a large amount of gas generation,so it has the hydrocarbon generation conditions for scale shale gas reservoirs.Second,speaking of the shale in this area,the content of felsic minerals is dominant,and matrix pores and micro fractures are more developed to provide some reservoir spaces.Moreover,the content of brittle minerals is high,and the total content of illite and illite/montmorillonite interlayer in clay minerals is 94%.The shale of low expansion and brittle can be fractured easily.Third,fluid expansion generated in the process of continuous hydrocarbon generation is the main cause of overpressure in Es_(3).The shale interval with pressure coefficient higher than 1.2 presents an obvious anomaly in while-drilling gas logging and has the conditions for shale gas accumulation.Fourth,when the burial depth of Es_(3)in Qikou Sag is 3100-4400 m,shale gas and shale oil are associated mainly in the form of dissolved gas;when it is 4400-4850 m,wet gas is dominant;and when it is greater than 4850 m,dry gas is dominant.In conclusion,the middle-low slope area in Qikou Sag is a favorable target area for shale gas exploration,whose favorable exploration area is about 1476 km^(2) and shale gas resource is more than 1×10^(12) m^(3),indicating greater shale gas exploration potential.