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
Xu Jie1,2, Xiao Di1,2, Su Wen-Jie3, Yan Wei4, Zhong Shou-Kang1,2, Yang Ming-Yi5, Yang Meng-Ying1,2, Tan Xiu-Cheng1,2
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500,China; 2 Research Branch of Southwest Petroleum University,Key Laboratory of Carbonate Reservoirs,CNPC,Chengdu 610500, China; 3 No.3 Gas Production Plant of Changqing Oilfield Company,PetroChina,Xi'an 710018, China; 4 GWDC Mud Logging Company,CNPC,Liaoning Panjin 124010, China; 5 Exploration Division,Changqing Oilfield Company of PetroChina,Xi'an 710018, China
Abstract：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.
Xu Jie,Xiao Di,Su Wen-Jie et al. 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[J]. JOPC, 2022, 24(2): 261-277.
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