Reservoir variations and heterogeneities under valley‐monadnock palaeo‐geomorphology in hinterland area of the Junggar Basin, northwestern China: Sedimentary characteristics and diagenetic alterations

Abstract

Palaeo‐geomorphology can exert significant effects on reservoir quality via controlling the distribution of depositional facies and diagenetic alterations. However, little attention has been paid to the studies of the reservoir variations and heterogeneities under distinctive valley‐monadnock palaeo‐geomorphology. Taking the Lower Cretaceous Qingshuihe Formation (K1q) in the Junggar Basin as an example, this study improves the current understanding of reservoir potential within topography of this kind by examining the link between depositional facies, diagenesis and their impact on reservoir quality. In total, nine types of lithofacies can be identified in the K1q reservoirs, which can be further grouped into lowstand systems tract (LST) gravelly braided river and debris flow deposits and transgressive systems tract (TST) sandy braided river delta deposits. These reservoirs are rich in rock fragments and characterized by moderate porosity and moderate to low permeability with development of intergranular and dissolution pores. Generally, detrital composition has a greater impact on reservoir quality of the LST conglomerates than on that of the TST sandstones. However, texture maturity only shows positive correlation with physical properties of the sandstone reservoirs. The mechanical compaction was responsible for the major destruction of the initial porosity. However, the large heterogeneities in calcite cements account for the extremely heterogeneous reservoir quality. Calcite cements formed during eodiagenesis usually show poikilotopic fabric and preferentially concentrated inthe basal coarser units/facies within inidividual fining‐upward successions. Consequently, the upper finer units/facies became more favourable for the secondary dissolution and oil charging processes during mesodiagenesis. In contrast, the amounts of authigenic quartz and clay minerals are relatively low. Nevertheless, the contents of different authigenic clay minerals show a variety of relationships with the reservoir quality. The valley‐monadnock topography spatially controls the scales of the LST and TST reservoirs. The simultaneously evolutionary processes between the back‐filling of the valleys and weathering of the monadnocks resulted in the prolonged residence time and large area of depositional water contacting with the weathering crust. This may give rise to the formation of alkaline pore fluids and facilitate the nucleation and growth of eogenetic carbonate cements and grain‐coating chlorite. Furthermore, the reservoir quality generally displays an increasing trend from the monadnock to the valley thalweg due to the increase of hydrodynamic strength and the less influence of mélange debris flow deposits.