Multigenic formation models of the Cambrian-Ordovician Dolomites in Northern and Central Tarim Basin

Key words: 
• Lithofacies /
• geochemical characteristics /
• dolomitization /
• early Paleozoic /
• Tarim Basin

Abstract: [Objective] The genesis of dolomites is still controversy, and dolomite reservoirs play an important role in carbonate oil and gas exploration, whose reservoir properties (porosity and permeability) are largely influenced by the genesis types and texture characteristics of dolomites. In recent years, many large oil and gas fields have also been found in domestic dolomite reservoirs, such as Tarim Basin, Sulige Gas Field in Ordos Basin, Puguang Yuanba Gas Field in Sichuan Basin and Anyue Large or Super Large Gas Field. Therefore, systematic researches on the texture types and genetic mechanisms of dolomites yield profound theoretical significance, and also will promote greater advance in carbonate oil and gas exploration in China. The Tarim Basin shares large volume of oil and gas reserves, the exploration target horizons within which has gradually shifted from medium shallow- to medium-buried layers to ultra-deep layers. Especially, industrial oil and gas flows were encountered in the deeply-buried dolomites of the Lower Paleozoic in the Tarim Basin, making the formation, evolution and reservoir characteristics of deep dolomite reservoirs in this basin become the focus of scholars' research. However, the formation mechanism of the Cambrian-Ordovician dolomites in the Tarim Basin has not yet reached a consensus due to deep burial depth, complex genesis and difficult exploration of these dolomites. This study increases the understanding of their origin and provides theoretical support for oil and gas exploration in the region. [Methods] Detailed petrographic and geochemical (trace-rare earth elements, stable carbon and oxygen isotopes and 87Sr/86Sr ratios) studies of these dolomites were conducted on the Cambrian-Ordovician dolomites of northern and central Tarim Basin based on the classification scheme of dolomite proposed in [25, 26]. [Results and Conclusions] According to the occurrence of dolomites, the Cambrian-Ordovician dolomites were divided into matrix and cement dolomites. Based on grain sizes, contact relationship between crystal planes (plane or curved surface), and crystal shape (euhedral, subhedral or anhedral), six types of dolomite structures were further identified for the matrix dolomite: (1) very fine to fine crystalline dolomite (Md1); (2) relict mimetic dolomite (Md2); (3) very fine to fine crystalline, planar-e(s) floating dolomite (Md3); (4) fine crystalline, planar-e(s) dolomite (Md4); (5) fine to coarse crystalline, nonplanar-a dolomite (Md5); and (6) coarse crystalline, nonplanar saddle dolomite (Md6). The distribution patterns of rare earth elements (REE) (slight enrichment or depletion of light REE, weak Eu negative or positive anomalies, and weak Ce negative or positive anomalies), δ13C values (-2.83–1.72‰; average -1.64‰) and 87Sr/86Sr values (0.7087–0.7116; average 0.7095) in the six matrices are similar to Cambrian-Ordovician micritic limestone and contemporaneous seawater. The diagenetic fluids of Md1 and Md2 are coeval seawater with varying degrees of evaporation and concentration, and were formed by (pene)contemporaneous dolomitization (including sabkha and reflux infiltration dolomitization) in relatively restricted depositional environments. The parent fluids of Md3 and Md4 are residual seawater, created by shallow- to medium-burial dolomitization. The diagenetic fluids of Md5 are variants of contemporaneous water, formed by deep-burial dolomitization or recrystallization of earlier dolomites. Deep hydrothermal fluids were responsible for the Md6 formation as a result of strong water–rock interactions with the host dolomites, influenced by mutual regulation and re-equilibration between the deep hydrothermal fluids and surrounding rocks.