  Taking $r>0$, let $\pi_{2r}(x)$ denote the number of prime pairs $(p, p+2r)$
with $p\le x$. The prime-pair conjecture of Hardy and Littlewood (1923) asserts
that $\pi_{2r}(x)\sim 2C_{2r} {\rm li}_2(x)$ with an explicit constant
$C_{2r}>0$. There seems to be no good conjecture for the remainders
$\omega_{2r}(x)=\pi_{2r}(x)- 2C_{2r} {\rm li}_2(x)$ that corresponds to
Riemann's formula for $\pi(x)-{\rm li}(x)$. However, there is a heuristic
approximate formula for averages of the remainders $\omega_{2r}(x)$ which is
supported by numerical results.
