Characteristics and Genesis of Dolomites from the Devonian Guanwushan Formation in the Jiguanshan Section, southwest Sichuan Basin, China

Abstract The Guanwushan Formation of Middle Devonian in Sichuan Basin is an excellent oil and gas reservoir, but its genetic mechanism is still unclear, and previous studies mainly focus on the typical profile in northwest Sichuan. In this paper, the characteristics and genesis of dolomite in Guanwushan Formation are studied by taking the Jiguanshan section in southwest Sichuan as an example. The section of Jiguanshan in Chongzhou City is located in the middle of Longmen Mountain and the southwest of Chengdu. Through field profile measurement, petrological observation, isotope analysis, REE distribution mode analysis, fluid inclusion analysis, the following understandings are obtained. 1. The Guanwushan Formation of the profile is in conformity with the overlying Shawozi Formation and the underlying Jinbaoshi Formation; 2. The dolomite types developed in Guanwushan Formation in the profile mainly include reef dolomite, and “zebra” structure dolomite can be seen. 3. On the profile, a large number of organisms can be observed, mainly including tetragonal corals, transverse corals, cladocoropsis, etc. Under the microscope, corals with intact diaphragms can be observed on thin slices; 4. The Jiguanshan section is a platform margin facies deposit, located on the continental margin side of the carbonate platform; 5. The carbon and oxygen isotope contents in the samples are all within the distribution range of carbon and oxygen isotopes of seawater in the same period of the Devonian system. It is speculated that most of the dolomite in the study section are reef quasi contemporaneous dolomite, samples’ δ 18 O content can be divided into three types of dolomite according to different temperatures; The distribution pattern of rare earth elements in the sample shows obvious δCe negative anomaly, speculated to be Devonian sea water, or slightly modified Devonian sea water quasi syngenetic origin, with obvious zebra dolomite filling δEu positive anomaly is a typical hydrothermal dolomite; The inclusion homogenization temperature test shows that there are two different origins of dolomite on the profile.