四川盆地震旦纪—早寒武世克拉通内裂陷地质特征

随着四川盆地震旦系—寒武系天然气勘探取得重大突破,地质认识也有了新的变化,其中之一就是在川中高石梯一磨溪地区西侧发现了一个近南北向展布的负向构造带,但对其构造性质、边界特征与展布范围、沉积充填特征、形成时期与演化等方面的认识还存在着较多分歧。由于该负向构造带与安岳震旦系寒武系特大型气田的形成关系十分密切,因此有必要对其进行深入研究。通过分析该区震旦系—寒武系钻井资料和野外剖面观察,利用27 000 km二维地震资料和最新钻井资料(高石17井),结合该盆地区域构造背景,研究了绵竹—长宁克拉通内裂陷边界与展布、地层沉积特征和演化过程。结果表明:①该裂陷总体呈南北向展布,裂陷东边界陡,相对稳定发育,裂陷西边界缓且不同时期发育位置不同;②震旦纪灯二期末该裂陷表现为近似对称的坳陷形态特征,分布面积为3×10~4 km~2;③灯四期末其又表现为受裂陷东边界断层控制的东陡西缓的断陷形态特征,分布范围达8×10~4 km~2;④该裂陷演化分为震旦纪灯一期一灯二期坳陷雏形形成、灯三期—灯四期裂陷形成发育、早寒武世麦地坪期—筇竹寺期裂陷充填与沉降、早寒武世沧浪铺期—龙王庙期裂陷萎缩与消亡等4个阶段。结论认为:该裂陷两侧震旦系—寒武系天然气成藏条件优越,是该区今后的主要勘探方向。

Geological characteristics of the Sinian-Early Cambrian intracratonic rift,Sichuan Basin

The Mianzhu - Changning intracratonic rift was found in the western part of Gaoshiti – Moxi area in the middle Sichuan Basin and was closely related to the formation of the giant Sinian – Early Permian gas field in the Anyue Block. Based on the log and outcrop data of 27 000 km seismic sections and the latest drilling data of Well Gao17, the boundary, distribution, deposition characteristics and evolution history of the Mianzhu - Changning intracratonic rift were systematically studied. The following findings were obtained. (1) This rift was mainly situated in the NS direction with a steep and stably-developed eastern boundary and a gentle western boundary developed at different periods and locations. (2) At the end of the second member of the Dengying Fm of the Sinian strata (Z₂dn₂), the rift, covering an area of 30 000 km², was characterized as an approximately symmetrical depression. At the end ofZ₂dn₄, it was characterized as an 80 000 km² east-steep and west-gentle half graben controlled by faults in the east. (3) The sedimentary thickness ofZ₂dn within the rift was thin, which consisted mainly of dark shales and micritic dolomites of shelf facies. On the contrary, the sedimentary thickness ofZ₂dn on the eastern and western sides of the rift was thick with ultra thick carbonate rock, where platform margin reefs and grain banks were developed. (4) In ∈₁m-∈₁q(Early Cambrian Maidiping – Qiongzhusi period), black charcoal shale and politic siltstone of deep-water shelf facies were deposited; and microclasticrock-carbonate rock of hybrid shelf facies and relatively low-energy grain-beach dolomite were deposited in ∈₁c and ∈₁l (Early Cambrian Canglangpu and Longwangmiao periods) respectively. (5) The evolution of the rift was primarily divided into four stages, i.e.,Z₂dn₁ -Z₂dn₂ embryonic stage,Z₂dn₃ -Z₂dn₄ developing stage, ∈₁m-∈₁q filling and subsidence stage, and ∈₁c-∈₁l shrinkage stage. In conclusion, both sides of this rift provide favorable conditions for gas accumulation and pooling in the Sinian – Early Cambrian strata in this study area and will become the main gas exploration target in the future.