鄂尔多斯盆地东缘奥陶系马家沟组四段豹斑状云质灰岩特征及成因: 以关家崖剖面为例*

鄂尔多斯盆地东缘中奥陶统马家沟组发育豹斑状云质灰岩,因其分布广泛、成因复杂以及具有重要的油气储集意义而长期受到关注。文中以山西省兴县关家崖剖面马家沟组四段顶部为例,依据宏观、微观岩石学特征,结合碳氧稳定同位素、原位激光剥蚀的微量稀土元素等地球化学分析手段,分析了豹斑状云质灰岩的特征及其成因。研究表明,豹斑状云质灰岩的特征为: (1)常发育于(含颗粒)泥晶灰岩中,且向上白云石化程度增强,云质斑块内常伴有针状或板条状石膏假晶;(2)按产状可大致分为水平、斜交和不规则花斑3种类型,分别对应于向上变浅序列的下部、中部和上部,序列顶部多见近原地角砾化;(3)基质灰岩与云质斑块的δ¹³C、δ¹⁸O值较同期海水略显负偏,顶部不规则云质斑块负偏程度最大;(4)“豹斑”与基质灰岩均具有较低的Mn、Sr、Ba等含量和低Mn/Sr值;(5)“豹斑”的ΣREE含量较基质灰岩略低,且皆表现为轻稀土富集、重稀土亏损的稀土配分模式,δCe微弱负异常,δEu无异常。依据上述结果,推测关家崖剖面马四段豹斑状云质灰岩产状受到生物扰动和高频海平面驱动的早成岩期岩溶耦合控制,序列由下至上耦合改造由弱变强,形成下—中部以岩溶影响较小的水平和斜交虫迹为特征、上部叠合岩溶强改造形成不规则花斑的垂向序列组合;早成岩期的暴露岩溶不仅叠合改造生物潜穴,而且蒸发浓缩的有限重卤水沿叠合优势通道优先交代孔渗条件更好的区域,至埋藏时期白云石化作用进一步加强,从而形成了区内类型多样的豹斑状云质灰岩。研究结果还揭示出沉积期微地貌控制的早成岩期岩溶作用是马四段这类特殊的豹斑状云质灰岩或豹斑状云岩储集层形成的关键,且在很大程度上控制了这类储集层的分布,这一认识可为高频暴露驱动的早成岩期岩溶研究提供新的材料。

Characteristics and genesis of leopard-spotted dolomitic limestone in the Member 4 of Ordovician Majiagou Formation:a case study from Guanjiaya section in eastern margin of Ordos Basin

The leopard-spotted dolomitic limestone are found in the Middle Ordovician Majiagou Formation in the eastern margin of the Ordos Basin. It has been concerned by sedimentologists and reservoir geologists due to the extensive development,complex genesis,and important reservoir significance. This paper takes the top of the Member 4 of the Majiagou Formation in Guanjiaya Section loacted at Xingxian of Shanxi Province as an example. Based on the macroscopic and microscopic petrological characteristics,the geochemical analysis including the stable carbon and oxygen isotopes and in-situ laser ablation of trace rare earth elements,the characteristics and genesis of leopard porphyritic dolomitic limestone are analyzed. The results show that(1)Leopard-spotted dolomitic limestone often develops in granular-bearing micrite limestone,and the degree of dolomitization is enhanced upward. The needle-like or lath-like gypsum pseudocrystals often develop in dolomitic patches;(2)Leopard-spotted dolomitic limestone can be roughly divided into three types based on orientation of rocks: Horizontal,oblique and irregular variegation,corresponding to the lower,middle and up parts of the upward shallowing sedimentary sequence. The near-situ brecciation is often found at the top of the sedimentary sequence;(3)The δ¹³C and δ¹⁸O of the matrix limestone and dolomitic patches are slightly negatively biased compared with the seawater of the same period,and the top irregular dolomitic patches are the most negatively biased;(4)Leopard spot and matrix in limestone both have low contents of Mn,Sr,Ba and other trace elements,and low Mn/Sr ratio;(5)The total REE content of the leopard spotted part is slightly lower than that of the matrix part of limestone,and the REE distribution pattern is characterized as enriched in LREE and depleted in HREE,with the slight negative δCe anomaly and no δEu anomaly. Comprehensive analysis shows that the occurrence of leopard porphyritic dolomitic limestone in the Member 4 is controlled by the coupling of eogenetic karsts driven by biological disturbance and high-frequency in changes of sea level. The coupling modification is from weak to strong level from bottom to top of the sedimentary sequence. The bottom-middle part of the sequence is characterized by horizontal and oblique insect traces that are less affected by the karst,and the upper superimposed karst is strongly transformed to form a vertical sequence combination of irregular mottling. The exposed karst in the early diagenesis period overlaps and transforms the biological burrows. The evaporative and concentrated heavy brine will preferentially replace the areas with high porosity and permeability along the overlapping dominant channels. The dolomitization during the burial period is further strengthened,thus forming various types of leopard porphyritic dolomitic limestone. The research results also indicate that the eogenetic karstification controlled by the micro-topography during the sedimentary period is the key to the formation of the special leopard porphyritic dolomitic limestone or leopard porphyritic dolomite reservoirs of the Member 4 of Majiagou Formation,and this type of reservoir. This understanding also provides new materials for the study of eogenetic karsts driven by high-frequency of exposure periods.