Abstract
The 400m thick Lower Permian carbonate sequence of South Sichuan/China, which is considered to be a most important gas reservoir rock, exhibits calcitized gas bubbles and dolomitized burrows which are the result of early diagenesis.
The fossiliferous, highly algal-enriched limestones are part of the Yangtze Platform and are composed of biomicrites, biosparites and calcarenites of subtidal and intertidal environments which underwent a maximum burial depth of some 6000 m.
The calcitized or, rarely, chert-filled gas bubbles represent a morphogenetic series ranging from a slightly brecciated sediment to globular forms and vertically elongated to branched dm-sized vugs; the series contains some internal sediments (lumps, calcarenites) which are identical to the host rock.
Vadose influence is missing. Marcasite aggregates precipitated at the beginning of calcite cement formation emphasize the reducing conditions whereas negative δ13C values (−0.6 to −2.5 ‰) indicate the participation of CO2 and H2O originating from the decay of organic matter. Calcite cements contain reduced amounts of Na. K, Sr, Ba and Mg but are enriched in Fe and Mn in comparison to the host rock.
It is obvious from the data that rising biogenic gases (CH4, CO2, NH3, H2S) under different partial pressures formed the vugs within an unconsolidated sediment (water content approximately 80%).
Burrows parallel to the bedding (typeThalassinoides), which contain biocalcarenites (echinoderm bioclasts) as internal sediment, underwent early diagenetic dolomitization. These dolomites, which rarely developing into dm-thick supratidal beds, document a slight evaporation within the intertidal environments. Surprisingly enough they reveal a slight over abundance of Mg (Ca47Mg53...) which can be explained by an influence of rising formation waters rich in Mg. In comparison to the host rock the dolomites are enriched in Fe, Mn, K, Na, Zn, Pb and As, and exhibit decreased amounts of Ba and Sr as well as slightly higher δ13C values.
The vugs play an insignificant role in respect to reservoir properties, because of their isolated distribution and early cementation which destroyed the pore spaces. By contrast, the recrystallized early diagenetic dolomitized burrows exhibit intercrystalline pores which are often occupied by migrated bitumen, but they are not economically relevant for the reservoir properties of the whole sequence.
Zusammenfassung
In einer etwa 400 m mächtigen unterpermischen Karbonatabfolge Süd-Sichuans/China, die als wichtiger Erdgasspeicher gilt, treten calcitisierte Gasblasen und dolomitisierte Grabgänge als frühdiagenetische Bildungen auf.
Die zur Yangtze-Plattform gehörende Sequenz weist fossilreiche (meist algenreiche) Biomikrite,-sparite und Biokalkarenite sub- und intertidaler Environments aus, die eine maximale Versenkungstiefe von 6000 m erfuhren.
Die calcitisierten oder seltener mit Chert gefüllten Gasblasen lassen von kugeligen über vertikal ausgelängte zu verzweigten Formen eine im dm-Bereich kontinuierliche morphogenetische Reihen erkennen, die mit dem Wirtsgestein identische Internsedimente (lumps, Calcisiltit) enthält.
Während Hinweise auf vadose Einflüsse fehlen, weisen zu Beginn der Calcitzementabscheidung gebildete Markasit-Aggregate auf reduzierende Bedingungen und negative δ13C-Gehalte (−0.6 bis −2,5 ‰) auf eine Beteiligung von CO2 und H2O hin, die bei der Zersetzung organischer Substanz entstanden.
Der Calcit zeigt eine Abreicherung von Na, K, Sr, Ba, Mg und eine Anreicherung von Fe und Mn gegenüber dem Wirtsgestein.
Die ermittelten Daten machen wahrscheinlich, daß aufsteigende biogene Gase (CH4, CO2, NH3, H2S) im noch weichen Kalkschlamm (Wassergehalt ungefähr 80 %) die beschriebenen Hohlräume bei unterschiedlichen Partial-drucken erzeugten.
Schichtgebundene Grabgänge (TypThalassinoides), die Biokalkarenite (Echinodermen) als Internsedimente enthalten, unterlagen einer frühdiagenetischen Dolomitisierung. Diese Dolomite, die sich nur selten zu dm-mächtigen supratidalen Lagen entwickeln, sind Ausdruck geringer Eindampfung in intertidalen Environments; sie weisen jedoch einen leichten Mg-Überschuß (Ca47Mg53...) auf, der durch eine zusätzliche aszendente Mg-Zufuhr zu erklären ist.
Gegenüber dem Wirtsgestein sind die Dolomite durch Fe, Mn, K, Na, Zn, Pb und As an- und durch Ba und Sr abgereichert und außerdem durch höhere δ13C-Gehalte unterschieden.
Als Porenraumlieferanten für thermales Methan spielen die Gasblasen auf Grund ihres isolierten Auftretens und dem frühen Zeitpunkt ihrer Zementation keine Rolle. Die volumenmäßig in unbeschränkten Umfang auftretenden rekristallisierten, frühdiagenetisch dolomitisierten Grabgänge weisen Interkristallin-Poren auf, die meist auch mit Bitumen gefüllt sind. Diese Poren sind für die Speichergesteinsentwicklung der gesamten Sedimentfolge ohne Bedeutung.
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Xiaozeng, L., Schneider, W. & Wenbin, T. Biogenic methane and burrowing as important controlling factors in the early diagenesis of permian carbonate rocks in South Sichuan/China. Facies 18, 289–301 (1988). https://doi.org/10.1007/BF02536803
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DOI: https://doi.org/10.1007/BF02536803