This paper is the result of oil and gas exploration engineering.

Objective Carbonate strata are susceptible to sea-level changes and thus develop high-frequency sequences. However, the accuracy of geochemical indicators reflecting sea-level changes is insufficient, resulting in poor understanding about the high-frequency sea-level fluctuation. The controls of sea-level changes on the high-frequency sequence and sedimentary evolution of the platform-margin reef-shoal deposits remain unclear.

Methods Continuous core data from a well in the Upper Ordovician Lianglitage Formation in the Tarim Basin were selected for microfacies analysis, carbon and oxygen isotope analyses by densely-collected samples.

Results Seven microfacies types developed in the middle and upper part of Lianglitage Formation. The analysis of the sedimentary evolution shows that the sedimentary environment evolved from tidal flat to reef and shoal, and to deeper platform during the middle to late period of deposition. The carbon and oxygen isotope values ranges from: δ¹³C is 0.5993 ‰-1.6228 ‰ (average 1.139 ‰) and δ¹⁸O value is -8.3608 ‰- -5.1452 ‰ (average -6.790 ‰). The changes of the δ¹³C and δ¹⁸O correspond well with the evolution of microfacies and sedimentary cycles. The analysis of paleo-ocean conditions shows that the Lianglitage Formation was deposited under warm climate and the reef and shoal samples recorded high paleo-temperature, representing a high rate of carbonate production. The paleo-salinity reflected by the Z-value is high according to the samples at the bottom of the tidal flat, and it is relatively low at the top of high-frequency cycles, probably indicating the influence of meteoric water. The range of δ¹³C and δ¹⁸O values differ from the Tazhong area, the southern margin of Tabei area, and Bachu area, which is mainly related to the difference in environment and water depth.

Conclusions At least three cycles of sea-level change and two secondary cycles exit in the middle to upper parts of the Lianglitage Formation. Different levels of sea-level changes are mainly driven by paleoclimate changes that control the formation of high-frequency sequences. Sea level rises slowly and then continues to be stable or oscillate on a small scale, which provide a favorable condition for the deposition and development of thick-bedded bioclasts and carbonate sands. Sea level rose sharply during the late sedimentary period of the Lianglitage Formation, and the sedimentary environment turned into inter-shoal lagoons and deeper platforms. The early-stage karstification caused by the small-scale sea-level drop at the top of the high-frequency sequence is an important factor to improve the reservoir quality of reef and shoal faces.