Sequential logic implies that previous inputs effect current outputs, which given the feedback you have in the circuit you do have. On the other hand when most people (including me) think of sequential logic they picture logic that is registered with state and clocked (e.g. you have flip flops). What you have here is sequential logic, but it's asynchronous (you don't have a clock input). At first glance I would have wrongly called this combinational since I just looked for a register/flip flop somewhere, but because you have feedback your output depends on both current inputs and previous inputs.
To see this assume for a second that a=1, b=0, d=1, c=x. This means that your feedback path (the second input to the or gate with c as input) is driven high. Z0 is now 1. Assume now that you toggle a low. Z0 will still be high. That means that Z0 is dependent both on current inputs as well as previous inputs. Now for fun set d=0. Now you've cleared Z0.
From wikipedia: "In digital circuit theory, sequential logic is a type of logic circuit whose output depends not only on the present value of its input signals but on the sequence of past inputs, the input history.1 This is in contrast to combinational logic, whose output is a function of only the present input. That is, sequential logic has state (memory) while combinational logic does not. Or, in other words, sequential logic is combinational logic with memory."