Stratigraphic Analysis of Upper Cretaceous Rocks in the Mahajanga Basin, Northwestern Madagascar: Implications for Ancient and Modern Faunas

Upper Cretaceous strata of the Mahajanga Basin, northwestern Madagascar, yield some of the most significant and exquisitely preserved vertebrate fossils known from Gondwana. The sedimentology of these strata and their stratigraphic relations have been the focus of renewed geological investigations during the course of five expeditions since 1993. We here designate stratotypes and formalize the terrestrial Maevarano Formation, with three new members (Masorobe, Anembalemba, Miadana), and the overlying marine Berivotra Formation. The Maevarano Formation accumulated on a broad, semiarid alluvial plain bounded to the southeast by crystalline highlands and to the northwest by the Mozambique Channel. The Berivotra Formation was deposited in an open marine setting that evolved from a clastic- to a carbonate-dominated shelf, resulting in deposition of the overlying Betsiboka limestone of Danian age. New stratigraphic data clearly indicate that the Maevarano Formation correlates, at least in part, with the Maastrichtian Berivotra Formation, and this in turn indicates that the most fossiliferous portions of the Maevarano Formation are Maastrichtian in age, rather than Campanian as previously reported. This revised age for the Maevarano vertebrate assemblage indicates that it is approximately contemporaneous with the vertebrate fauna recovered from the Deccan basalt volcano-sedimentary sequence of India. The comparable age of these two faunas is significant because the faunas appear to be more similar to one another than either is to those from any other major Gondwanan landmass. The revised age of the Maevarano Formation, when considered in the light of our recent fossil discoveries, further indicates that the ancestral stocks of Madagascar's overwhelmingly endemic modern vertebrate fauna arrived on the island in post-Mesozoic times. The basal stocks of the modern vertebrate fauna are conspicuously absent in the Maevarano Formation. Finally, the revised age of the Maevarano Formation serves to expand our global perspective on the K/T event by clarifying the age of a diverse, and arguably the best preserved, sample of Gondwanan vertebrates from the terminal Cretaceous.     

粤北丹霞盆地晚白垩世丹霞组沉积相及古气候意义

粤北丹霞山作为世界自然遗产"中国丹霞"的典型代表,在地貌方面已经积累了丰富的研究成果,但是对丹霞地貌成景地层(晚白垩世红层)的沉积环境和古气候的研究较少。通过对巴寨剖面丹霞组的野外观测、砾石统计、室内砂岩样品薄片鉴定及粒度分析,进行详细的沉积相研究。巴寨剖面丹霞组出露巴寨段和锦石岩段,前者以砾岩为主,发育粒序层理及平行层理、底侵蚀面、叠瓦状构造等,而后者以发育大型交错层理的风成砂岩夹古土壤为特征。在盆地其他位置,可见丹霞组白寨顶段砾岩地层。研究结果表明,在巴寨剖面的晚白垩世丹霞组红层中,可以识别出正粒序层理砾岩、逆粒序层理砾岩、平行层理砾岩、无沉积构造砾岩、大型交错层理风成砂岩、中—薄层砂岩、古土壤7种岩相。巴寨段砾石大小主要为0.5～9.5 cm,成分以脉石英、石英砂岩及砖红色砂岩为主。锦石岩段砂岩粒径为0.03～0.5 mm,分选性好,以跳跃组分为主,指示风成环境。丹霞组从巴寨段到锦石岩段沉积物粒度变细、沉积速率由快变慢,指示了由河流主导的冲积扇沉积体系向沙漠沉积体系的转变,反映了古气候向干热变化,锦石岩段的风成砂岩可能与东亚中纬度地区晚白垩世Campanian期古气候广泛的干旱化有关。丹霞组沉积末期(白寨顶段),古气候条件再次变湿,降水增加,河流将粗粒碎屑物再次搬运到盆地发生沉积,新的冲积扇叠覆在锦石岩段风成砂岩—古土壤序列之上。     

渤海湾盆地歧口凹陷沙河街组一段下亚段湖相白云岩及其与海相白云岩的差异

渤海湾盆地歧口凹陷古近系沙河街组一段下亚段湖相白云岩主要为颗粒白云岩和泥晶白云岩两大类,常与泥质岩成薄互层或与其他碳酸盐岩互层产出。该湖相白云岩有序度较低,类似于海相准同生白云岩;Mg/Ca值偏离正常的化学计量值,具有富钙特征,说明其为快速交代的产物;碳氧同位素、Z值分析结果则反映了湖水的咸化程度较高。根据低海平面期蒸发浓缩盐水在湖底形成密度层并向灰质沉积层中回流渗透导致白云石化发生的机理,建立了歧口凹陷沙一下亚段湖相白云岩的成因模式。湖相白云岩的分布特征及其与海相白云岩的差异分析表明,湖相白云岩具有分布面积小、白云石化作用不彻底等特征,油气储集性能较为局限。     

新疆塔里木盆地库车坳陷北缘白垩系储层沉积相研究

库车坳陷北缘白垩系储层主要分布在巴西盖组和巴什基奇克组。白垩纪时期,库车坳陷发育了不同类型的三角洲沉积体系,巴西盖期为湖泊三角洲沉积体系;巴什基奇克早期主要为扇三角洲沉积体系;巴什基奇克中晚期主要为湖泊-辫状三角洲沉积体系。垂向上,巴西盖组下部砂岩段总体表现为加积序列,上部泥岩段整体表现为向上砂质增多的进积序列;巴什基奇克组下部第Ⅲ岩性段表现为进积-加积演化序列特征;巴什基奇克组中上部Ⅱ-Ⅰ段总体上构成一个向上变粗的进积序列。平面上,从巴西盖期至巴什基奇克晚期,库车坳陷北缘沉积相总体上呈近东西向展布,南北相带变化明显的特点。在沉积相研究的基础上,结合构造活动和成岩作用分析认为,研究区内白垩系巴什基奇克组中上部辫状三角洲前缘砂体为优质储层,以克拉2井区最好,向西至克拉1井,向东至克拉3井储层均变差。     

The Vendian succession of northeastern Spitsbergen: Petrogenesis of a dolomite-tillite association

The sedimentary record of late Precambrian time is magnificently displayed in the highland snowfields of northeastern Spitsbergen (Svalbard). Vendian strata are represented essentially by the Polarisbreen Group which consists mostly of dolostone and includes two dolomitic glacial units. The oldest sediments in the Polarisbreen Group compose the Elbobreen Formation (c. 400 m), which is divided into four laterally-persistent members. The Lower Carbonate Member (E1, 125 m) contains a distinctive basal dark-grey limestone (with microspar-filled synaeresis cracks) suggested to be of lagoonal origin and associated with minor dolostone, shale and chert. Higher parts of the member are dominantly dolostone, partly stromatolitic, with some shale and sandstone; shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite. Penecontemporaneous dolomite is partially overprinted by microsparry dolomite, thought to be of groundwater origin.The redefined Petrovbreen Member (E2) consists of diamictite and other detrital dolostone. Pronounced thickness variations (2–40 m) are thought to be original depositional features. The member represents the deposits of a short glacial period in which the following depositional processes are inferred: lodgement (massive diamictite), subaqueous meltout (massive and bedded diamictite), ice-rafting (lithologies bearing dropstones, and possibly also diamictite), redeposition by sediment gravity flows (some diamictite and conglomerate; rhythmite and shale), current winnowing (thin tabular conglomerate), subaerial or subaqueous meltwater action (channelled conglomerate and sandstone), periglacial shrinkage (diamictite wedge-fillings).The MacDonaldryggen Member (E3, 230 m) is a monotonous succession of shaly dolostone of lagoonal origin. It grades up into the Slangen Member (E4, 25 m) which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment.The Wilsonbreen Formation (160 m) represents a return to glacial deposition, but this time longer-lasting and with substantial extra-basinal material represented. The Gropbreen Member (W1, 28–107 m) and the Ormen Member (W3, 44–139 m) consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a Middle Carbonate Member (W2, 3–30 m) which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone. The same depositional processes can be recognised as in the Petrovbreen Member, but the Wilsonbreen Formation is overall of somewhat more continental aspect (lower proportion of rhythmite and dropstone structures). In addition there are: basal breccia and crack-fillings formed by frost-shattering of the underlying cemented dolostone, tabular sandstone thought to be formed by wave reworking of outwash, a striated (terrestrial) cobble pavement, supraglacially-derived breccia horizons, periglacial wedges filled by sand and the W2 assemblage of possible lacustrine origin.The Dracoisen Formation (525 m) represents an abrupt return to non-glacial conditions. An upward-deepening wave-dominated succession of pure dolostone (D1, 20 m) and impure dolostone (D2, 105 m) is succeeded by offshore black shale (D3, 150 m) and then by a very-shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics (D5, 10 m) separating dolomitic sandstone and shale (D4, 80 m and D6, 150 m). The contact with the transgressive Cambrian sandstones above is poorly exposed.Deposition of the succession dominantly under marine conditions is inferred, but it is difficult to rule out a lacustrine environment at any particular horizon. This dolomite—tillite association can be explained by penecontemporaneous (and minor secondary) dolomite formation in marginal environments (with warm climatic indicators at some levels) being sharply interrupted, because of rapid climatic changes, by glacial sediments containing abundant detrital dolomite. Since the latter sediments make up only 17% of the 1080m-thick succession, glacial conditions only occupied a small proportion of Vendian time.     

鄂尔多斯盆地北部侏罗系直罗组沉积特征与演化*

鄂尔多斯盆地东北部发现的多个砂岩型铀矿床均赋存于侏罗系直罗组下段砂岩中。前人对已知铀矿床分布区直罗组的沉积学研究程度相对较高,但对盆地北部直罗组大区域沉积体系展布与演变、物源供给特征等的研究仍较为薄弱。文中在大量钻井资料分析、野外剖面实测等基础上,将盆地北部砂岩型铀矿含矿层段直罗组下段细分为2个亚段。在直罗组中识别出河流和三角洲相沉积,认为直罗组下段下亚段主要发育砾质、砂质辫状河沉积,东北部地区发育辫状河三角洲沉积;直罗组下段上亚段主要发育砂质辫状河和曲流河沉积;直罗组上段则以曲流河沉积为主。结合前人研究工作,认为源岩物质组成、有利沉积相带和气候条件对鄂尔多斯盆地北部砂岩型铀矿的成矿均具有重要控制作用。对盆地北部直罗组沉积特征及其演化的整体认识,可为该区砂岩型铀矿床的进一步勘查提供重要的沉积学依据。     

鄂尔多斯盆地东部下二叠统山西组山2段成岩相划分及展布

鄂尔多斯盆地东部下二叠统山西组山2段岩石类型主要为石英砂岩、岩屑质石英砂岩和岩屑砂岩,储集层经历了压实、压溶、硅质胶结、碳酸盐胶结交代、高岭石胶结、杂基蚀变以及溶蚀等多种成岩作用。在成岩作用研究的基础上,通过大量的岩心观察和薄片鉴定,结合阴极发光、探针分析等方法,将研究区划分出7种单因素成岩相,即：强压实、压溶-石英次生加大、蚀变高岭石、沉淀高岭石、杂基充填、碳酸盐胶结交代、不稳定组分溶蚀成岩相,在此基础上对其进行平面叠加,确定了利于储集层发育的优势成岩相。其中压溶-石英次生加大与杂基、假杂基溶蚀作用的综合作用（叠加和改造）决定了山西组山2段有利的成岩相带,在陕141井区、榆37井区、子洲-清涧地区山西组山2段形成了天然气优质储集层。     

潮水盆地下白垩统庙沟组铀的预富集及其找矿意义

潮水盆地早白垩世经历了干旱型、相对潮湿型、干湿型古气候的演变过程。庙沟组下段主要由冲积扇 扇前平原辫状河沉积体系构成 ,上段则以曲流河或河湖交替沉积为特征。干湿型气候条件下“相对滞流环境”有利于铀的聚集与浓缩 ,并主要以吸附铀和解吸铀的形式预富集在庙沟组上段厚层红色或红、灰相间的细碎屑岩及其孔隙水中。笔者认为在压实成岩过程中 ,上述预富集的铀逐渐排出并迁移到孔隙度较高、渗透性较好的砂岩中 ,构成砂岩型铀成矿的直接铀源 ,对庙沟组砂岩型铀成矿起着重要的控制作用。因此 ,笔者提出与厚层红色或红、灰相间的细碎屑岩直接接触的砂体是砂岩型铀矿找矿的重要对象     

广西百色盆地古近系始新统沉积相特征及演化

利用野外露头、地震、测井和分析化验等资料,对盆地古近系始新统那读组和百岗组进行层序地层和沉积相研究。那读组和百岗组可划分为3个Ⅲ级层序,9个体系域,那读组那三段构成层序Ⅰ,那读组那二段和那一段构成层序Ⅱ,百岗组构成层序Ⅲ。层序Ⅰ和层序Ⅱ是盆地断陷期沉积,发育冲积扇相、河流相、三角洲相、扇三角洲相、深湖亚相和滨-浅湖亚相。层序Ⅱ湖泊沉积范围广,厚度大,厚层的湖相暗色泥岩是盆地主要烃源岩和区域性盖层。层序Ⅲ是盆地坳陷期的沉积,发育冲积扇相、扇三角洲相、河流相、三角洲相和滨-浅湖亚相。盆地中古近系沉积经过了洪积或冲积扇、河流三角洲→湖泊（扇三角洲）→河流的发育过程,反映了盆地从初期形成→扩张→萎缩的完整的演化序列。在层序Ⅰ和层序Ⅱ的低水位体系域,田东凹陷的中央断凹、南部次凹和那笔凸起的北部断鼻等区带发育长轴分布的河流三角洲砂岩,这些砂岩处于烃源岩成熟区,油气成藏条件优越,是河道岩性油藏勘探的有利区。在层序Ⅱ湖侵体系域和高水位体系域,北部陡坡发育扇三角洲砂体,与烃源岩指状交叉接触,有利于油气运聚成藏,容易形成大量上倾尖灭岩性油藏。     

塔里木盆地西南地区白垩系沉积相特征

塔西南白垩系发育,可分为上、下两统。下白垩统克孜勒苏群可分4段,多以陆相沉积为主,富含棕红色砂砾岩夹少许砂岩、粉砂岩、泥岩和砾岩。上白垩统英吉沙群为海陆相并存,库克拜组可分2段,常见泥岩、膏岩和海相化石;乌依塔格组多为红色泥岩、泥质粉砂岩夹砂岩;依格孜牙组多见灰岩、白云质灰岩,富含海相化石;吐依洛克组为棕红色泥岩、石膏和砂、砾岩,含海相化石。通过勾勒9个岩性单元的沉积相展布,分析昆仑山前白垩纪的沉积环境演化过程。克孜勒苏群西区多为陆相快速堆积,东区远离陆源为三角洲和滨岸沉积,具有宽泛的冲积扇—辫状河三角洲相分布。库克拜组总体显示为辫状河三角洲—潮坪相变的过程;乌依塔格组以潮坪为主;依格孜牙组表现为碳酸盐岩台地—台地边缘的演化;吐依洛克组为宽广潮坪。     

渤海湾盆地黄河口凹陷古近系东营组三段沉积特征

综合利用地震、测井、录井、岩心及分析化验资料,通过综合分析和系统沉积学编图,确定了黄河口凹陷古近系东营组三段（东三段）主要发育扇三角洲、辫状河三角洲、湖泊和湖底扇。东三段下部辫状河三角洲发育于黄河口凹陷的西南部和南部,在渤南凸起的南北两侧均发育扇三角洲朵体,半深湖-深湖主要分布于西部深凹部位,在B-2断隆带南部的半深湖区发育湖底扇,绝大部分地带为滨浅湖沉积,以滨浅湖泥滩和混合滩沉积为主。东三段上部辫状河三角洲发育于黄河口凹陷的西南部、南部及D-4断隆带的南侧,仅在渤南凸起的北侧发育扇三角洲朵体,半深湖-深湖主要分布于西北次凹部位,绝大部分地带为滨浅湖沉积,以滨浅湖泥滩和混合滩沉积为主。E-1-1-F-4-1井一带继承性发育滨浅湖砂质滩坝沉积。滨浅湖砂质滩坝由含砾粗砂岩、含砾中砂岩、含砾细砂岩及中、细砂岩组成,骨架颗粒成分以石英为主,发育低角度交错层理,纹层由定向排列的砾石或暗色组分显示,纹层平直,以低角度相交。滨浅湖砂质滩坝微相区为东三段优质储集层发育区。     

Palaeoecological significance of turritelline gastropod-dominated assemblages from the mid-Cretaceous (Albian-Cenomanian) of Texas and Oklahoma,USA

The faunas of three previously poorly known and highly fossiliferous limestones from the upper Lower Cretaceous of Texas are dominated by turritelline gastropods. These faunas consist of turritelline-dominated assemblages in the Whitestone Limestone Member of the Walnut Formation in Travis County (middle Albian), the Keys Valley Marl Member of the Walnut Formation in Coryell County (middle Albian), and the Fort Terrett Formation in Kimble County (middle Albian). A fourth high-spired gastropod assemblage in the Segovia Formation in Pecos County (upper Albian) is not dominated by turritellines. Two other turritelline-dominated assemblages in non-carbonate rocks from the Albian and Cenomanian of Texas and Oklahoma are also described. These turritelline-dominated assemblage occurrences add considerably to our knowledge of the facies occurrence of Cretaceous turritelline-dominated assemblages, and they are consistent with the global facies distribution of these assemblages: i.e., although they are widespread in siliciclastic facies from Cretaceous to Recent, turritelline-dominated assemblages in carbonate facies occur almost exclusively in the Cretaceous and Paleogene.     

Petrography and geochemistry of the Middle Miocene Gebel El Rusas sandstones,Eastern Desert,Egypt: Implications for provenance and tectonic setting

Petrography and bulk rock geochemistry of the Middle Miocene sandstones of the lower and upper members of Gebel El Rusas Formation along the Egyptian Red Sea Coastal plain, have been investigated to determine the provenance, tectonic setting, and weathering condition of this formation. The Lower Member is formed mainly of sandstones and conglomerates with clay interbeds. The Upper Member is more calcareous and formed mainly of sandstones and limestones with marls and clays intercalations. Petrographically, the Lower Member sandstones are mostly immature and classified as arkoses with an average framework composition of \(\hbox {Q}_{66}\hbox {F}_{29}\hbox {R}_{5}\), and the Upper Member sandstones are partly submature (more quartzose, less feldspathic) and classified as subarkoses with an average framework composition of \(\hbox {Q}_{80}\hbox {F}_{17}\hbox {R}_{3}\). The Gebel El Rusas sandstones are enriched in Sr, Ba, Zr and Rb and depleted in Co and U, as compared to UCC. The chemical index of alteration (CIA) values suggest moderate weathering conditions. The geochemistry results revealed that the Gebel El Rusas sandstones were derived from felsic-granitic source rocks and deposited in a passive margin of a synrift basin. The inferred tectonic setting for Middle Miocene Gebel El Rusas sandstones in the study area is consistent with the regional geology of the Eastern Desert of Egypt during Middle Miocene.     

西藏边坝-洛隆地区下白垩统边坝组的建立及其意义

边坝组是新建立的一个岩石地层单位。该组与下伏下白垩统多尼组呈整合接触,与上覆上白垩统宗给组呈角度不整合接触关系。岩性以紫红色深灰色粉砂质泥岩夹薄层白云岩为特征,深灰色薄层粉砂质泥岩中产丰富的双壳类化石Trigonioides(Diversitrigonioides)xizangensis-Pleuromya spitiensis组合。边坝组的建立完善了该地区下白垩统地层系统,对冈底斯北缘弧后前陆盆地白垩纪岩相古地理研究具有重要意义。     

Depositional history and stratigraphical evolution of the Sakamena group (Middle Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Karoo Supergroup in Madagascar is subdivided into three lithostratigraphical units: the Late Carboniferous-Early Permian Sakoa Group; the Late Permian-Middle Triassic Sakamena Group; and the Triassic-Early Jurassic Isalo Group. The Sakamena Group is fairly well exposed in the southern Morondava Basin, where it is approximately 4000 m thick. The Sakamena Group is separated from the Sakoa Group by an angular unconformity. The Lower Sakamena Formation is characterised by two major facies associations: (1) interbedded muddy conglomerates and coarse sandstones; and (2) interbedded sandstones and mudstones, which were deposited in a rejuvenated rift setting by coarse-grained fluvial systems and debris flows on the rift margins. In the Vatambe area, facies represent fandelta deposition in a saline lake or tongue of the ocean. The Middle Sakamena Formation comprises three major facies: (1) laminated mudstones and sandstones; (2) sandstones; and (3) mudstones. The Middle Sakamena facies were deposited by low gradient meandering streams and in shallow lakes. The Upper Sakamena Formation was deposited in similar environments, except that it is comprised predominantly of red beds. The Isalo Group consists predominantly of coarse-grained sandstones (up to 6000 m thick). These sandstones were deposited by braided streams with the coarse detritus derived from a structural uplift in the east.     

Sedimentology,stratigraphy and palynological occurrences of the late Cretaceous Erlian Formation,Erlian Basin,Inner Mongolia,People's Republic of China

The Erlian Basin is one of the non-marine Cretaceous basins of north-east China that developed during the late Mesozoic continental extension in eastern Asia. This basin experienced two major tectonic events: (i) a syn-rift stage that was dominated by a fluvial–lacustrine depositional environment and (ii) a post-rift stage that was dominated by a fluvial environment. A new sedimentological study performed on Erlian Formation drill cores has led to the determination of an architectural model and to the subsequent characterisation of the stratigraphic evolution of this sedimentary unit during the late Cretaceous. The palynological occurrences that were identified in samples provided a possible stratigraphical age for the Erlian Formation.Sediments of the Erlian Formation occur at the top of the Cretaceous stratigraphic column of the Erlian Basin and were deposited during the post-rift stage. Facies architecture and the ideal succession of facies that were identified for this formation exhibit two different members, both dominated by a fluvial depositional environment: (i) the lower member, which is dominated by channels of a braided river system and (ii) the upper member, which is dominated by overbank deposits. The lower member expresses a tectonically induced uplift as indicated by channels clustering under negative accommodation, whereas a period of stratigraphic base-level rise that is associated with an increase of accommodation is identified in the upper member. Therefore the Erlian Formation highlights an alternation of short uplifts that were dominated by braided fluvial channel deposits with periods of stratigraphic base-level rise that were dominated by overbank deposits. This sedimentological architecture has significant metallogenic implications for the origin of confined permeable sandstone layers, which represent adequate host-rocks for roll front-type uranium deposits.The palynological assemblage Exesipollenites, Ulmipollenites/Ulmoideipites, Buttinia and Momipites that were recognised in two samples of the Erlian Formation has revealed a post-late Campanian age therefore more likely indicating a late Cretaceous age of deposition for the sediments of the Erlian Formation.     

新疆哈拉阿拉特山地区的地层和沉积环境

本文根据对新疆准噶尔盆地西北缘的哈拉阿拉特山地区的石炭系、二叠系所作的地层研究以所获的生物化石、岩矿特征、元素分布、粘土矿物组合等新资料,确定了本地区地层的时代、层序,修正了前人的认识。并对哈拉阿拉特山区的沉积环境进行了分析,探讨了沉积相演化过程。对准噶尔盆地的形成演化提出了见解。     

祁连山北缘旱峡早白垩世下沟组-中沟组沉积相研究

祁连山北缘旱峡地区发育完整的下白垩统下沟组-中沟组沉积序列;以陆源碎屑岩为主,发育砾岩、砂岩、钙质粉砂岩和页岩等;其中砾石分选差、磨圆一般、成分以灰白色砂岩为主,砂岩成分、结构成熟度中等-差。岩石组合、沉积序列和综合沉积特征研究表明,下沟组和中沟组主要由湖泊、扇三角洲、河流等3种类型沉积相、亚相与微相类型组成,发育了一系列重力流成因的沉积类型;湖泊沉积主要发育在下沟组底部和中沟组,扇三角洲沉积分布在下沟组中-上部,中沟组发育河流沉积。研究区早白垩世中期古流向以EN向为主,早白垩世晚期以W向为主,表明沉积沉降中心也由早期的旱峡西移至研究区西南侧。在综合分析的基础上,建立了研究区下白垩统下沟组-中沟组沉积相模式。     

四川盆地高石梯—磨溪地区震旦系灯影组储层特征及主控因素

经过数十年的勘探,勘探家们在四川盆地高石梯—磨溪地区获得了万亿方级的探明储量。为研究高石梯—磨溪地区震旦系(≈埃迪卡拉系)灯影组白云岩储层特征及成藏主控因素,开展了岩芯观察、薄片鉴定、阴极发光分析、物性测试、扫描电镜观察、主微量元素和碳氧同位素分析、包裹体均一温度测试、沉积相带划分、宏观构造对成藏控制作用解析等。结果表明:(1)高石梯—磨溪地区灯影组四段发育优质白云岩储层,包括泥晶藻云岩、砂屑白云岩和晶粒白云岩。(2)灯影组四段优质储集层遭受了桐湾Ⅱ幕的大气淡水岩溶作用和埋藏期热液改造作用,溶蚀孔隙广泛发育,平均孔隙度为4.8%,平均渗透率为0.5×10-3μm2,为"低孔—特低渗"型储层。(3)有利相带、白云石化作用控制了优质储层的发育和分布,多分布于裂陷槽边缘的台缘带内,尤其是藻丘滩相白云岩为研究区的主力产气层;埋藏期的热液白云石化作用改善了其储集物性和孔隙结构,主要证据包括:裂缝和溶蚀孔洞中可见各种热液矿物、溶蚀孔洞和裂缝中充填的鞍状白云石具有典型的雾心亮边结构、氧同位素组成"偏负"、流体包裹体均一温度平均值高达175.7℃、高含Mn和阴极发光下发明亮红光等。该研究成果对拓展古老碳酸盐岩油气勘探具有重要理论意义。