Study On Paleohydrodynamic Field In Middle Triassic In The West Of Sichuan Sunken And Potassium-rich Brine Mineralization Prediction

The deep underground brine is one of the objects on potash searching in China,there is the largest and rare underground brine which is distributed from Sinian toCretaceous stratum. Because of the ancient geological structure, paleoclimate,paleogeography, sedimentary environment and so on, the evaporite developed inmiddle late three Triassic in Sichuan Basin, and the world’s rare liquid potassium oreresource, potassium rich brine is stored in stratum. It has been found at Xuanhan,Suining, Qionglai and other places in petroleum exploration in Basin, and thepotassium content is the highest in the brine from Qionglai Pingluoba4well inWestern Sichuan sunken, which enrich in bromine, iodine, boron and other elements.It is the first large-scale multi-element comprehensive bittern deposit.Although the formers have a large number of research on potassium-rich brineorigin, but stay in water chemical data and lithology in single well of petroleum field,or physicochemical characteristics, don’t analysis formation and evolution ofpotassium-rich brine systematically and comprehensively.This article will discuss themineralization process of the potassium-rich brine at Pingluoba region, from theevolution of the paleohydrodynamic field in middle Triassicin the West of SichuanSunken, establish potassium-rich brine mineralization model, and point out favorablearea of potassium-rich brine in western Sichuan depression.First, the restoration of the paleohydrodynamic is based on in environmentalgeological conditions. In this paper, the author uses the previous research resultsabout tectonic and sedimentary evolution of the foreland basin in West Sichuan,reconstruct the environmental geological conditions in which hydrodynamic fielddeveloped and evolved since middle three Triassic, and divides into fourhydrogeological periods and five sub-periods. According to the type of groundwater system, hydrodynamic characteristics, the groundwater experienced the sedimentaryhydrogeological, leaching hydrogeological period, buried hydrogeological period(including three sub-periods) and structure hydrogeological period (including twosub-periods). And the author has a detailed analysis of the characteristics about thepaleohydrodynamic field in various hydrogeological period in West Sichuandepression, including to recharge runoff and drainage conditions, runoff intensity andwater storage capacity in underground water system.This paper analyzes the typical brine-bearing structure in Pingluoba area.Potassium-rich brine fisheries in the composite field structure, composed of aproductive layer and multiple reservoir cap assemblage in Longmen mountainpiedmont zone in west Sichuan. Potassium-rich brine is mainly stored in MiddleTriassic period four (T2l4) carbonate rock. Dissolved pores and cracks is the mainstorage space, the groundwater belongs to the karst-fissure water. Back-flushingreverse faults, thick gypsum layer and the high pressure belt from the static formationpressure compose the closed boundary in the brine-bearing structure system.Geochemical characteristics of potassium-rich brine and reservoir indicate thatbrine doesn’t only enrich in potassium and boron,but also contains rich traceelements, as highly metamorphosed brine.According to the evolution process of thepaleohydrodynamic field, it can be inferred that the water in the brine is from thefresh water in leaching, the water stored in original sedimentary, the water fromgypsum dehydration and the water released by the clay rock compaction; potassiumsources: residual sea water, solid sylvite, mung bean-rock and deep hydrothermal.Itcan establish the metallogenic model that gypsum-salt basin is the material basis forthe formation of potassium-rich brine; palaeohydrodynamic field is dynamiccondition in which the brine accumulated the potassium; tectonic fissures are the mainchannel that brine migrated in; carbonate rock is good reservoir for brine; dissolvedfracture and dissolution pore is main storage space of the the brine; closed structuralhigh is important site for the potassium-rich brine enrichment; thick gypsum strata isthe closed lithology boundary for the brine.and point out that prediction system forbrine mineralization enrichment, made of three conditions includingpaleohydrodynamic field, lithofacies paleogeography and geology structure. Predictsthree class-level favorable area in West Sichuan depression area.The author adopts the viewpoint of groundwater system,， establishmineralization model and forecast system for potassium-rich brine based on theevolution process of palaeohydrodynamic. It will provide important basis for exploring and predicting deep underground potassium-rich brine in the unknownregions.