1000-0569/2006/022（08）-2171-81       Acta Petrologica Sinica   岩石学报

 

 		    
 

Diagenetic patterns and pore systems of the Lower Ordovician Majiagou Formation reservoirs of the central Changqing gas fields

 

WU YaSheng1, HE ShunLi2, LU Tao3,

DA ShiPan3, LI YuGang3, DAI YongDing1

 

 

1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029

2. China University of Petroleum, Beijing 102249

3. Research Institute of Petroleum Exploration and Development, Changqing Oilfield Company, Xi’an 710021, China

(Wu YS:   wys@mail.igcas.ac.cn)

 

 

Abstract    

The main Ordovician reservoirs of the central Changqing gas fields includes the eight beds of the M5 Member of the Early Ordovician Majiagou Formation: M511, M512, M513, M514, M521, M522, M533, and M541, in descending order. Beds M511, M512 and M513 are mainly finely-crystalline penecontemporaneous dolomites with abundant anhydrite nodules (generally 1~5 mm wide) indicating upper supratidal environments. Beds M514, M521, M522 are mainly laminated dolostones of upper intertidal environments. Beds M533 and M541 are mainly anhydrite rock with chicken-wire structure indicating continental sabkha environments and finely-crystalline dolomites with abundant anhydrite nodules (1~5 mm wide) indicating upper supratidal environments.

The most important diagenetic processes undergone by the M5 Member include: (I) precipitation of gypsum nodules in interstitial pores in the tidal-flat deposits due to evaporation and the penecontemporaneous dolomitization of the original calcium carbonate deposits; (II) partial or total replacement of the gypsum/anhydrite nodules by dolomites caused by mixing of seawater and freshwater; (III) dissolution of the gypsum in nodules by meteoric water and formation of nodule-moldic pores; (IV) partial or total filling of the nodule-moldic pores by fine, early calcites; (V) partial or total filling of the remaining pores by coarse, late calcites; and (VI) filling of the remaining pores by sylvite. Processes (I), (II) and (III) improved the reservoir quality, while (IV), (V) and (VI) reduced the reservoir quality.

Seven main diagenetic patterns were recognized from the M5 Member:   dolomitization –interstitial dissolution;  dolomitization-interstitial dissolution -calcite filling;  dolomitization -gypsum dissolution;  dolomitization -dolomite replacement -gypsum dissolution;  dolomitization –dolomite replacement –gypsum dissolution -early calcite filling;  dolomitization -dolomite replacement-gypsum dissolution-early calcite filling-late calcite filling; and  dolomitization-dolomite replacement-gypsum dissolution -early calcite filling -late calcite filling-sylvite filling. Dolomitization-gypsum dissolution, dolomitization -dolomite replacement-gypsum dissolution, and dolomitization–interstitial dissolution accounted for formation of the main reservoirs.

The main pore systems in the M5 Member include: (a) nodule-derived pores; (b) nodule-derived pores and nodule-swell fissures; (c) nodule-derived pores and intercrystalline pores; (d) gypsum crystal moldic pores; and (e) intercrystalline pores. Pore systems (a), (b) and (c) are the main types in this Member.

Gypsum nodules are abundant in beds M511, M512, M513, and M533, especially M541. Diagenetic processes (I), (II) and (III) occurred in beds M511, M512, M513, M514, M521, M522, M533, and M541, while (IV), (V) and (VI) mainly occurred in M514, M521, M522, and M541. Consequently, beds M512, M513, and M533 constitute the main reservoirs, while M521, M522, and M514 have relatively poor reservoir quality.

 

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Keywords: Ordos basin, Majiagou Formation, central Changqing gas fields, diagenesis, porosity system

 

 

 

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