In recent years,the Journal of Palaeogeography(Chinese Edition and English Edition) continually received some manuscripts. The authors of these manuscripts,according to the viewpoint and method of Miall's paper(1985),selected rocks from each bed in the clastic sections of their study areas and induced some rock types,such as conglomerates,sandstones and fine-grained stones,and considered them as lithofacies. It does not conform to the definition of lithofacies. I wrote some papers,i.e.,Feng(2018,2019,2020),to point out the problems and hope that the authors worldwide,especially Chinese authors,will not continually cite,spread and follow of viewpoint and method of Miall's paper(1985) blindly. Prof. Miall is the first person who considered rocks(in fact,the sediments)as lithofacies and proposed a facies analysis method. His viewpoint and facies analysis method confused the definition of facies and facies analysis method. My current paper is a special article to discuss the principal problems of Miall's paper(1985),i.e.,he considered the sediments as lithofacies and utilized lithofacies to analyze facies,but not to discuss the contributions and less strictness of architectural elements of his paper.Here,I have to declare that the facies in my current paper is the facies of sedimentary petrology,but not of other geological disciplines,such as igneous petrology,metamorphic petrology,palaeontology,stratigraphy,geophysics,geochemistry.Certainly,the definitions of facies and lithofacies are controversial. I will adhere to the policy of “A hundred flowers blossom and a hundred schools of thought contend” and hope that through academic discussions,contends and geological practice,these problems will be solved gradually.

Within the Middle and Upper Cambrian Xixiangchi Formation,large-scale breakthroughs of the oil and gas exploration never came true and only a small amount of productivity were obtained in the Weiyuan-Ziyang area. Hence,investigating the formation and associated controlling factors of those reservoirs can provide geological basis for the future hydrocargon exploration. Based on outcrops,cores,thin sections,as well as drilling and logging data of the central Sichuan Basin,this study made a detailed description for the characteristics of the Xixiangchi reservoir and then analyzed the influences of the shoal depositional combinations,eogenetic karstification and the telogenetic karstification on the reservoir. The results show that: (1)The Xixiangchi reservoir in the study area is mainly granular dolostone. Based on whether the inner texture of grains can be preserved,the granular dolomite is divided into residual clastic texture dolomite and powder-fine crystal dolomite,whose reservoir spaces come from intergranular dissolved pores,intercrystalline dissolved pores and vugs,and thus,those reservoirs are characterized by low porosity and low permeability as a whole;(2)There are three types of shoal depositional combinations in the Xixiangchi Formation,which are lagoon-shoal(-tidal flat),shoal-tidal flat or mixed tidal flat,and multi-stage shoal vertical overlapping,respectively. The multi-stage shoal vertical overlapping can form a stratal succession with the most grainstone,whose reservoir properties are the best;(3)The eogenetic karstification is crucial for the formation of the reservoir and the telogenetic karstification could also improve the reservoir quality.

The study on the sedimentary evolution of the Hetao Basin is relatively insufficient,especially for the Cretaceous and Paleogene strata in the Linhe depression,which has important oil and gas exploration potential. In this article,based on sedimentology,mineralogy,petrology,geochemistry,field and drilling data,we performed a systematic sedimentary research on the Cretaceous to Paleogene strata in the Linhe depression and to explore its petroleum significance. The results show that: (1)From the Cretaceous to Paleogene,the Linhe depression in the Hetao Basin was characterized by sub-tropical arid and semi-arid climate,an overall oxygen-rich water body but locally anaerobic-anaerobic,and alternation of fresh water and brackish water environments. (2)There are five types of sedimentary facies in the Cretaceous to Paleogene strata,including alluvial fan,braided river,braided river delta,fan delta,and lacustrine facies,which are further divided into 12 types of sedimentary subfacies and 22 types of sedimentary microfacies. In the northwestern depression,the alluvial fan-delta-lacustrine sedimentary system was developed due to strong fault activity and high topographic relief. In the southeastern depression,a relatively weak tectonic activity and low topographic relief resulted in a braided river-braided river delta-lacustrine sedimentary system. The sedimentary pattern of each period is similar and only shows the variation of the fan-lacustrine boundary. (3)The Cretaceous to Paleogene Linhe Formation was characterized by arid and semi-arid,shallow water,dual provenances controlled by weak episodic tectonic activity. In the early stage of basin development,the tectonic activity was strong,but the basin was small and shallow. The clastic materials from both sides of basin margin rapidly advanced into and even converged in the center of the basin,forming a basin filled up by sands. With the weakening of tectonic activity and the rapid rise of lake water in the late stage,the clastic materials supply was insufficient to fill the lacustrine basin,thus forming a lake facies. (4)Sedimentary facies distribution in the depression control the macro-scale size,superimposed relationship and vertical evolution of sand bodies,and further determine the spatial distribution of effective reservoirs. Under the dual controlling factors of sedimentary facies and faulting,the favorable reservoirs in the Cretaceous to Paleogene strata are mainly distributed in the central faulted belt,which is dominated by shore-shallow lake and delta front deposits.